Use mold for linking in CI tests

[zanieb]

Derived from #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)

[zanieb]

I think this is worth merging separately from #875. It'll be easier to tell if that's worthwhile once this lands.