checks: re-enable go mod tidy

go mod tidy should fail (e.g., make a network connection) only if it is
not up to date, which is exactly why this check was introduced
initially.

Change-Id: I55e79d21291fca02295aed9f5b94d156b30bfc15
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/493697
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
diff --git a/checks.bash b/checks.bash
index 799517d..9eed411 100755
--- a/checks.bash
+++ b/checks.bash
@@ -109,8 +109,7 @@
   check_integration
   check_headers
   go_linters
-  # TODO(zpavlinovic): find a way to disable only for non-longtest builders.
-  #go_modtidy
+  go_modtidy
 }
 
 usage() {