Only run checkgenerate on ubuntu

Shouldn't change based on OS, similar to how we have the lint step. And
add a simple name to the step.

Signed-off-by: Stefan VanBuren <[email protected]>

Author: stefanvanburen

.github/workflows/ci.yaml

@@ -87,7 +87,9 @@ jobs:
         run: go test ./...
         working-directory: protoc-gen-connect-python
 
-      - run: uv run just checkgenerate
+      - name: check running `just generate` does not create a diff
+        if: startsWith(matrix.os, 'ubuntu-')
+        run: uv run just checkgenerate
 
       - uses: codecov/codecov-action@v5
         if: ${{ matrix.coverage == 'cov' }}