Add gen-modcheck CI check

[jtraglia]

Mentioned here, it would be good to check that generator imports are correct. We can do this by running the generators with the new --modcheck flag. This is essentially a dry-run which returns before running any tests.

• electra: Epoch processing tests #3984 (comment)

Thanks to the check_mod function, this will throw an error if we forget to update a generator.

• Add new check_mods function for generators #3970

I wanted to use --collect-only but it was broken. I added a "fix" but it didn't work because the KZG generators do things differently and would still execute. My solution was to replace --collect-only.