bazel: reject symlink outputs from cachable actions

The remote caching protocol doesn’t know about symlinks, and the remote spawn strategy currently converts all symlink into regular files when uploading to the cache. This behavior is insidious because local execution preserves symlinks. So, local and remote execution may yield different results if symlinks are involved:

$ mkdir /tmp/my-cache
$ cat BUILD
genrule(
    name = 'emit-symlink',
    outs = [':a', ':b'],
    cmd = '''
touch $(location :a)
ln -s a $(location :b)
''',
)
$ bazel build --experimental_local_disk_cache --experimental_local_disk_cache_path /tmp/my-cache :emit-symlink
$ ls -lh bazel-genfiles/
total 0
-r-xr-xr-x 1 benjamin benjamin 0 Mar 13 02:35 a
lrwxrwxrwx 1 benjamin benjamin 1 Mar 13 02:35 b -> a
$ bazel clean --expunge
$ bazel build --experimental_local_disk_cache --experimental_local_disk_cache_path /tmp/my-cache :emit-symlink
$ ls -lh bazel-genfiles/
total 0
-r-xr-xr-x 1 benjamin benjamin 0 Mar 14 02:36 a
-r-xr-xr-x 1 benjamin benjamin 0 Mar 14 02:36 b

Handling symlinks in remote caching seems tricky. (What if the link target is absolute?) Therefore, I propose that trying to upload a symlink output to the cache should be an error.