Install GAMS using iiasa/actions for CI

[khaeru]

This PR uses the new iiasa/actions repo and setup-gams action to install GAMS on GitHub Actions runners.

This approach is more modular and simple than the prior approach of checking out the ixmp repo and calling a script contained there.

Other changes:

• Also use the ts-graphviz/setup-graphviz action, which does essentially the same thing as the Bash script used from ixmp
• Remove code in message_ix.testing.nightly that handles the GAMS license upload and download. The new action uses license file contents stored as a repository secret.

How to review

No review; CI changes only.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A; CI changes only
• Update release notes.

[codecov]

Codecov Report

Merging #452 (0e1c9ea) into master (9efac12) will increase coverage by 0.0%.
The diff coverage is 90.0%.

❗ Current head 0e1c9ea differs from pull request most recent head a568e5e. Consider uploading reports for the commit a568e5e to get more accurate results

@@          Coverage Diff           @@
##           master    #452   +/-   ##
======================================
  Coverage    93.3%   93.3%           
======================================
  Files          37      37           
  Lines        2798    2800    +2     
======================================
+ Hits         2611    2613    +2     
  Misses        187     187           

Impacted Files	Coverage Δ
message_ix/tools/add_year/__init__.py	66.2% <66.6%> (+0.1%)	⬆️
message_ix/tests/reporting/test_computations.py	100.0% <100.0%> (ø)
message_ix/tests/test_core.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_addon.py	100.0% <100.0%> (ø)
...age_ix/tests/test_feature_bound_activity_shares.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_bound_emission.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_price_commodity.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_price_emission.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_storage.py	92.4% <100.0%> (ø)
message_ix/tests/test_macro.py	100.0% <100.0%> (ø)
... and 2 more

[khaeru]

The Windows tests appear to all pass, but the step is failing:

XFAIL message_ix/tests/test_core.py::test_add_horizon[args7-kwargs7-None]
XFAIL message_ix/tests/test_core.py::test_add_horizon[args8-kwargs8-None]
==== 113 passed, 2 deselected, 5 xfailed, 4 warnings in 444.20s (0:07:24) =====
Error: Process completed with exit code 1.

Needs further investigation.

[khaeru]

Needs further investigation.

I'll temporarily exclude this in order to unblock other PRs, e.g. #451, and then open a new PR to re-enable and debug it.