copyright: limit copyright checking to .go files

The existing code looked at any file ending 'go' including,
for instance, default.pgo.

Change-Id: I9a8344d2e23da351ca13a888a367457537e5c1b1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/622895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

copyright/copyright.go

@@ -52,7 +52,7 @@ license that can be found in the LICENSE file.`)
 
 func checkFile(toolsDir, filename string) (bool, error) {
 	// Only check Go files.
-	if !strings.HasSuffix(filename, "go") {
+	if !strings.HasSuffix(filename, ".go") {
 		return false, nil
 	}
 	// Don't check testdata files.