Bump GAMS version in nightly CI worklow

[glatterf42]

Similar to iiasa/message-ix-models#250 (review), I'm hoping today's nightly CI test failure also just arises from an incompatibility of using an older GAMS version but a newer GAMS license for this kind of run. This PR thus bumps the GAMS version in the nightly workflow and temporarily enables them to make sure the fix works.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Update tests; coverage checks both ✅
• ~[ ] Add, expand, or update documentation. ~ Just CI.
• ~[ ] Update release notes. ~ Just CI.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.6%. Comparing base (618e13a) to head (3c16a74).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix/tests/test_nightly.py	0.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #892     +/-   ##
=======================================
- Coverage   95.6%   95.6%   -0.1%     
=======================================
  Files         46      46             
  Lines       4335    4337      +2     
=======================================
  Hits        4147    4147             
- Misses       188     190      +2     

Files with missing lines	Coverage Δ
message_ix/tests/test_nightly.py	0.0% <0.0%> (ø)

[glatterf42]

I've been looking at events that trigger CI runs and I've stumbled upon pull_request_target. If I'm not mistaken, this can close #603 since the nightly tests will now automatically run on every PR that changes any GAMS file (.gms, .gpr, and .gdx for now).

[glatterf42]

This test seems to not be considered for our coverage report, so new lines here are considered untested. I'd argue this should not stop this PR, though.

[glatterf42]

Without this, our secrets could be extracted by any workflow changing GAMS files.

[glatterf42]

Note: the tests passed before, so I disabled them again. They won't run when we apply a label to this PR because it would need to clear both filters, i.e. change some GAMS files and get a label to run.