Only run go mod vendor if git differences

[rambleraptor]

go mod vendor is an expensive operation that sometimes crashes due to intermittent network issues.

If there's no difference in the commit, it doesn't look like there's any reason that go mod vendor should be run.

---

[all]

[terraform]

[terraform-beta]

[ansible]

[inspec]

[rambleraptor]

From the Concourse run:

Old mm-generate run-time: ~7m
New mm-generate run-time (when no TF changes detected): 5m40s

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
This PR seems not to have generated downstream PRs before, as of e44725a.

Pull request statuses

No diff detected in terraform-provider-google-beta.
No diff detected in terraform-provider-google.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I didn't open any new pull requests because of this PR.

[rambleraptor]

@ndmckinley - Magician code review
@SirGitsalot - just for your context.

[nat-henderson]

Niiiice. Thank you, great call.