Address GAMS licensing for PRs

[khaeru]

The repo currently uses a GAMS license stored as a GitHub Actions encrypted secret. This means it cannot be used in PRs from forks. See e.g. #20.

To do: adjust the configuration so that PRs from forks can have passing checks.

Some options, not all mutually exclusive:

1. Don't create a gamslice.txt file if the secret is not available.
2. Use an older version of GAMS (29 or older).
3. Use pytest marks and GHA configuration selectively disable checks on PRs from forks.
4. Allow usage of secrets for PRs from forks #50

[glatterf42]

I guess this can be closed thanks to #281 and #288 :)

[khaeru]

Agreed, thanks for following up.