fix: conditionally include proxy flags

[tomasgareau]

I think #51 may have introduced a bug in the installers for python or ccppr_vsc -- when proxy was not set (or left to the default false), DIInstall python tried to call:

python3 -m venv .
bin/python -m pip install debugpy --proxy false

causing an error since --proxy false is not a valid argument for pip.

I suspect this is also the cause of #58.

This PR fixes that by conditionally adding the proxy flags (i.e., if proxy is "" or nil, it won't add any proxy flags to the commands).