ci(mac): use classic linker for gridgen with gnu compilers

[wpbonelli]

• fix gridgen gcc/g++ compile errors with -Wl,-ld_classic to select the classic linker
  • https://forums.developer.apple.com/forums/thread/737707
• there are still failures building gridgen with intel on macos-14, it looks like a failure to find the appropriate header files — and there is some separate issue building GSFLOW

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.183%. Comparing base (51313b0) to head (fcb637b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #173 +/- ## ============================================= + Coverage 75.638% 79.183% +3.545% ============================================= Files 20 20 Lines 3526 3526 ============================================= + Hits 2667 2792 +125 + Misses 859 734 -125 ``` [see 5 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/pymake/pull/173/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)