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)

.github/workflows/ci.yaml

@@ -62,6 +62,7 @@ jobs:
             3.12
       - name: "Install Rust toolchain"
         run: rustup show
+      - uses: rui314/setup-mold@v1
       - name: "Install cargo insta"
         uses: taiki-e/install-action@v2
         with: