Run the check-mode CI job only on Ubuntu

And not on macOS on Windows.

(If it is to be kept as a separate job, then possibly caching
should be enabled for it. But I suspect it can be moved into
another existing job, most likely the `test` job, which already
runs a number of distinct checks via `just`.)

.github/workflows/ci.yml

@@ -360,11 +360,7 @@ jobs:
         run: cd gix-pack && cargo build --all-features --target "$TARGET"
 
   check-mode:
-    strategy:
-      matrix:
-        os: [ ubuntu-latest, macos-latest, windows-latest ]
-
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v4