Use Depot 16-core runner for testing on Linux (#14460)

Reduces Linux test CI to 1m 40s (16 core) or 2m 56s (8 core) to from 4m
25s. Times are approximate, as runner performance is pretty variable.

In uv, we use the 16 core runners.

.github/workflows/ci.yaml

@@ -115,7 +115,7 @@ jobs:
 
   cargo-test-linux:
     name: "cargo test (linux)"
-    runs-on: ubuntu-latest
+    runs-on: depot-ubuntu-22.04-16
     needs: determine_changes
     if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
     timeout-minutes: 20