Run go mod tidy --diff in CI

[nathanjcochran]

Runs go mod tidy --diff in the CI pipeline, to ensure that the go.mod and go.sum files correctly reflects the modules needed by the source code (i.e. no missing modules and no extra unused modules). This is basically a way to make sure people run go mod tidy after adding/removing dependencies.

Per go help mod tidy:

The -diff flag causes tidy not to modify go.mod or go.sum but
instead print the necessary changes as a unified diff. It exits
with a non-zero code if the diff is not empty.

[nathanjcochran]

Moved this up from below because it's extremely fast, and better to give fast feedback on the install script than make people wait for all the tests to run.