workflows: improve replace check in go.mod

Since the latest comment introduces a retract statement, with a comment
that contains `replace', the check failed, so we improve it to only
consider `replace' if they're the first thing on a line.

.github/workflows/go-test.yml

@@ -32,7 +32,7 @@ jobs:
 
       - name: Check that go.mod does not contain a replace
         run: |
-          if grep -q "replace" <go.mod; then
+          if grep -Eq "^\s*replace" <go.mod; then
             echo "go.mod contains a replace but should not." >&2
             false
           fi