Downgrade the build-only-sysroot check step

Looks like this is failing on Ubuntu 24.04, the new default of
`ubuntu-latest`, so downgrade it to have it get fixed in a separate PR.

.github/workflows/main.yml

@@ -165,7 +165,7 @@ jobs:
 
   build-only-sysroot:
     name: Build only sysroot
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
         with: