duckdb: http_proxy and https_proxy env vars are ignored when installing extensions.

What happens?

I am trying to install the httpfs extension executing INSTALL 'httpfs'; using a proxy, but setting the environment variables do not work.

Note: Using python request library to download the extension works for me:

 r=requests.get("http://extensions.duckdb.org/662041e2b/linux_amd64/httpfs.duckdb_extension.gz",
     headers={"User-Agent":"DuckDB 0.3.4 662041e2b linux_amd64"},
     proxies={'http': '{my http proxy', 'https': 'my https proxy'},
     timeout)

Is there any other way to define what proxy to use when downloading extensions?

To Reproduce

1. set then environment variables http_proxy and https_proxy
2. Execute INSTALL 'httpfs'
3. duckdb does not use the proxy to download the extension

Environment (please complete the following information):

• OS: Linux
• DuckDB Version: 0.3.4
• DuckDB Client: Python

Before Submitting

• Have you tried this on the latest master branch?

• Python: pip install duckdb --upgrade --pre
• R: install.packages("https://github.com/duckdb/duckdb/releases/download/master-builds/duckdb_r_src.tar.gz", repos = NULL)
• Other Platforms: You can find binaries here or compile from source.

• Have you tried the steps to reproduce? Do they include all relevant data and configuration? Does the issue you report still appear there?