ci(dep): Check only for go.mod file (#1397)

This commit is to update go-mod github action step to check for only
go.mod file. Ideally, both go.mod and go.sum should be checked, however,
the current behavior of `go mod tidy` command will not clean up old
entries in go.sum.

Additionally, dependentbot is currently failing as there is dirty change
in go.sum file.

Author: sayboras

.github/workflows/pr.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Check go mod
         run: |
           go mod tidy
-          git diff --exit-code
+          git diff --exit-code go.mod
 
   # We already run the current golangci-lint in tests, but here we test
   # our GitHub action with the latest stable golangci-lint.