CI: Prevent the introduction of `git =` dependencies

[Oliver Tale-Yazdi]

Release is currently blocked since we added some git dependencies recently: paritytech/merkle-mountain-range#3

Should add a CI check that prevents their addition.

[Lulu]

This would fit into to parity-publish check command that already runs in CI

[Oliver Tale-Yazdi]

Now done by https://github.com/paritytech/polkadot-sdk/blob/master/.github/scripts/deny-git-deps.py.