github: Build the pinned Ubuntu jobs on 22.04

This makes the prebuilt toolchains require 22.04.

The GitHub Actions Ubuntu 20.04 images are deprecated and are
being removed soon, see
https://github.com/actions/runner-images/issues/11101.

.github/workflows/msvcrt.yml

@@ -47,7 +47,7 @@ jobs:
   # Build a new toolchain with a different CRT choice.
   linux-msvcrt:
     needs: [prepare]
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
         with: