[red-knot] mypy_primer: larger depot runner (#17547)

## Summary

A switch from 16 to 32 cores reduces the `mypy_primer` CI time from
3.5-4 min to 2.5-3 min. There's also a 64-core runner, but the 4 min ->
3 min change when doubling the cores once does suggest that it doesn't
parallelize *this* well.

.github/workflows/mypy_primer.yaml

@@ -26,7 +26,7 @@ env:
 jobs:
   mypy_primer:
     name: Run mypy_primer
-    runs-on: depot-ubuntu-22.04-16
+    runs-on: depot-ubuntu-22.04-32
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2