Only run release workflows for upstream

[hugovk]

When pushing main on a fork, the release workflow triggers but, as expected, fails to deploy to PyPI because API tokens are missing from forks:

ERROR    HTTPError: 403 Forbidden from https://upload.pypi.org/legacy/          
         Invalid or non-existent authentication information. See                
         https://pypi.org/help/#invalid-auth for more information.              

Before: https://github.com/hugovk/isort/actions/runs/3719681768/jobs/6308688909

Let's skip the release workflow for forks.

After: https://github.com/hugovk/isort/actions/runs/3719767321

Also, whilst the release drafter does work for forks, let's only run that for upstream too.

[codecov]

Codecov Report

Merging #2052 (f501d7c) into main (6c83bcc) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2052   +/-   ##
=======================================
  Coverage   98.80%   98.80%           
=======================================
  Files          38       38           
  Lines        3084     3084           
  Branches      745      745           
=======================================
  Hits         3047     3047           
  Misses         26       26           
  Partials       11       11