Add a submodule check to CI #18019

Add a check to the CI that checks whether the repository has any git submodules (either directly or of subtrees) and rejects it if so.

In #17398 I accidentally almost added three submodules because they are part of the crc32c third-party repository, which was subtree'd there. This was only avoided because the gitian build failed.

We do not make use of submodules so there doesn't need to be any exception.

Useful skills:

• Shell scripting

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This should be added to ./test/lint, right? Edit: I guess, inlining it somewhere in ./ci/test/ also works.

Sirs. I would like this request. Is there a bounty given for this request?

Is there a bounty given for this request?

No.

I implemented this in #18056

I would like this request too.

Done in #18056.