Ignore _gopath in verify scripts

The _gopath directory is used in Jenkins to install extra dependencies,
and any files in it should be ignored by the various verification
scripts.

hack/update-gofmt.sh

@@ -36,6 +36,7 @@ find_files() {
       \( \
         -wholename './output' \
         -o -wholename './_output' \
+        -o -wholename './_gopath' \
         -o -wholename './release' \
         -o -wholename './target' \
         -o -wholename '*/third_party/*' \