virtualenv: 20b1: "RuntimeError: failed to detect cpython3.6.8-64|..."

this is breaking some tests for pre-commit

this is similar to #1514 but involves absolute paths

here’s a minimal reproduction:

set -euxo pipefail
rm -rf venv1 venv2
virtualenv venv1
venv1/bin/pip install virtualenv --upgrade --pre
"$PWD/venv1/bin/python" -mvirtualenv venv2 -p "$PWD/venv1/bin/python"

$ bash t.sh 
+ rm -rf venv1 venv2
+ virtualenv venv1
Using real prefix '/usr'
Path not in prefix '/home/asottile/opt/venv/include/python3.6m' '/usr'
New python executable in /home/asottile/workspace/pre-commit/venv1/bin/python3
Also creating executable in /home/asottile/workspace/pre-commit/venv1/bin/python
Installing setuptools, pip, wheel...
done.
+ venv1/bin/pip install virtualenv --upgrade --pre
Collecting virtualenv
  Using cached virtualenv-20.0.0b1-py2.py3-none-any.whl (4.6 MB)
Collecting appdirs<2,>=1.4.3
  Using cached appdirs-1.4.3-py2.py3-none-any.whl (12 kB)
Collecting filelock<4,>=3.0.0
  Using cached filelock-3.0.12-py3-none-any.whl (7.6 kB)
Collecting importlib-metadata<2,>=0.12; python_version < "3.8"
  Using cached importlib_metadata-1.4.0-py2.py3-none-any.whl (29 kB)
Collecting importlib-resources<2,>=1.0; python_version < "3.7"
  Using cached importlib_resources-1.0.2-py2.py3-none-any.whl (32 kB)
Collecting six<2,>=1.12.0
  Using cached six-1.14.0-py2.py3-none-any.whl (10 kB)
Collecting zipp>=0.5
  Using cached zipp-2.1.0-py3-none-any.whl (4.6 kB)
Installing collected packages: appdirs, filelock, zipp, importlib-metadata, importlib-resources, six, virtualenv
Successfully installed appdirs-1.4.3 filelock-3.0.12 importlib-metadata-1.4.0 importlib-resources-1.0.2 six-1.14.0 virtualenv-20.0.0b1 zipp-2.1.0
+ /home/asottile/workspace/pre-commit/venv1/bin/python -mvirtualenv venv2 -p /home/asottile/workspace/pre-commit/venv1/bin/python
Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/__main__.py", line 27, in <module>
    run()
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/__main__.py", line 16, in run
    run_via_cli(args)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/run/__init__.py", line 21, in run_via_cli
    session = session_via_cli(args)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/run/__init__.py", line 35, in session_via_cli
    CreatorSelector(interpreter, parser),
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/run/plugin/creators.py", line 15, in __init__
    creators, self.key_to_meta, self.describe, self.builtin_key = self.for_interpreter(interpreter)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/run/plugin/creators.py", line 26, in for_interpreter
    meta = creator_class.can_create(interpreter)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/create/via_global_ref/builtin/via_global_self_do.py", line 33, in can_create
    for src in cls.sources(interpreter):
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/create/via_global_ref/builtin/via_global_self_do.py", line 54, in sources
    for host_exe, targets in cls._executables(interpreter):
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/create/via_global_ref/builtin/cpython/common.py", line 31, in _executables
    host_exe = Path(interpreter.system_executable)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/discovery/py_info.py", line 203, in system_executable
    result = self.find_exe_based_of(inside_folder=env_prefix)
  File "/home/asottile/workspace/pre-commit/venv1/lib/python3.6/site-packages/virtualenv/discovery/py_info.py", line 225, in find_exe_based_of
    "failed to detect {} in {}".format(what, os.pathsep.join(possible_folders))
RuntimeError: failed to detect cpython3.6.8-64|cpython3.6.8|cpython3.6-64|cpython3.6|cpython3-64|cpython3|cpython-64|cpython|CPython3.6.8-64|CPython3.6.8|CPython3.6-64|CPython3.6|CPython3-64|CPython3|CPython-64|CPython|CPYTHON3.6.8-64|CPYTHON3.6.8|CPYTHON3.6-64|CPYTHON3.6|CPYTHON3-64|CPYTHON3|CPYTHON-64|CPYTHON|python3.6.8-64|python3.6.8|python3.6-64|python3.6|python3-64|python3|python-64|python|PYTHON3.6.8-64|PYTHON3.6.8|PYTHON3.6-64|PYTHON3.6|PYTHON3-64|PYTHON3|PYTHON-64|PYTHON in /usr/bin:/usr

$ uname -a
Linux asottile-MacBookPro 4.15.0-74-generic #84-Ubuntu SMP Thu Dec 19 08:06:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.3 LTS
Release:	18.04
Codename:	bionic
$ venv1/bin/python --version --version
Python 3.6.8 (default, Oct  7 2019, 12:59:55) 
[GCC 8.3.0]
$ venv1/bin/pip freeze --all
appdirs==1.4.3
filelock==3.0.12
importlib-metadata==1.4.0
importlib-resources==1.0.2
pip==20.0.2
setuptools==45.1.0
six==1.14.0
virtualenv==20.0.0b1
wheel==0.34.1
zipp==2.1.0