Use go mod tidy --diff to validate go.mod

The "-diff" flag was added in Go 1.23 and does the right thing in a
single command.

Author: cbandy

.github/workflows/test.yaml

@@ -20,7 +20,7 @@ jobs:
         with: { go-version: stable }
 
       - name: Ensure go.mod is tidy
-        run: go mod tidy && git diff --exit-code -- go.mod
+        run: go mod tidy --diff
       - name: Ensure generated files are committed
         run: make check-generate
       - run: make check