Selectively update snapshots

[edmundmiller]

Idea

Extend the current functionality to allow --update-snapshot versions so you can only update a specific snapshot named "versions".

Use case

We're working on implementing automated dependency bumping in nf-core/modules. Ideally we'd like to bump the versions in the environment.yml, and then bump the version snapshots with something like nf-test test --only-changed --update-snapshot versions and then run a seperate nf-test test --ci to make sure none of the expected outputs changed.

In the meantime, I think we should only snapshot the versions in the stub workflows, and then give them a tag "stub" and we can then only run the snapshot update in CI with --tag stub