Docs release gh-action doesn't use the cached python environment

[rickstaa]

Currently, the docs release gh-action is not using caching (see this run). Caching in the Mac and Linux Github actions seems to be working fine (see this run). I will leave this for now but open a question on the cache/issues page of github community forum when the BLC package is public. Only difference I could find is that the Os related actions use a python matrix where the other actions use a hardcoded python value:

matrix:
  python-version: [3.6, 3.7, 3.8] # Supported python versions
...
with:
   python-version: ${{ matrix.python-version }}

vs

with:
    python-version: 3.7