Don't run GCM build on trivial PRs

[mathomp4]

Description

This PR should update the CI such that PRs labeled with "0 diff trivial" will not need a full build of GEOSgcm, but just do the cheaper (though perhaps not needed) MAPL build and tests. The hope is that people will only label trivial changes that are trivial, though this could back fire.

Motivation and Context

The GEOSgcm test is expensive. Until we can run these builds at make -j20 say on our own runners, it's probably best if trivial PRs don't need that.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)

Checklist:

• I have tested this change with a run of GEOSgcm (if non-trivial)
• I have added one of the required labels (0 diff, 0 diff trivial, 0 diff structural, non 0-diff)
• I have updated the CHANGELOG.md accordingly following the style of Keep a Changelog