Add PR check for whether go mod tidy produces a diff

[eversC]

I think go build will need to be checked in the same way

The goreleaser job in circleci is quite flakey, I'm sure there's various reasons, including go versions being used by people locally that doesn't match that of the circleci jobs.

If after running go mod tidy or go build there are changes to go.mod or go.sum, the check should fail (as eventually go releaser would end up failing when a new tag is pushed and the release circleci job runs.