Use `mold` for linking in CI tests (#887)
Derived from https://github.com/astral-sh/puffin/pull/875
This gets us a significant speedup.
I would not read the commits individually. I can squash them but they
were used for testing various scenarios.
### Test compile times
Ranges are the lowest and highest I've seen. Huge variability in GitHub
Actions runners.
**Before:**
7m 21s - 8m 22s (cold cache)
110s - 120s (warm cache)
**After:**
6m 15s - 7m 05s (cold cache)
57s - 70s (warm cache)
**Improvement:**
4% - 25% (cold cache)
36% - 52% (warm cache)
