Ensure gems directory is linted with Gemspec/AvoidExecutingGit

What does this MR do and why?

Previously we only checked vendor/gems for invocations of git in the gemspec files. !121676 (merged) introduced the gems directory, so we should lint this directory as well.

Relates to #415739 (closed)

How to set up and validate locally

1. Run scripts/lint-vendored-gems.sh. It should pass.
2. Revert !124141 (merged).
3. Run scripts/lint-vendored-gems.sh again. It should now fail.

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