lints workflow : msrv sometimes needs a crate directory

Because those repos contain multiple crates we want to check the msrv for, they need a custom msrv job that `cd`s into the crate's root directory:
- https://github.com/lurk-lab/bellpepper-gadgets
- https://github.com/lurk-lab/bellpepper

This makes the msrv step in the linting job from this repository unusable.

See https://github.com/lurk-lab/bellpepper-gadgets/pull/13
See also #7, more specifically the suggestion of a crate parameter.

Start with the lints workflow and its MSRV step, then read issue #7 and the referenced bellpepper-gadgets pull request. Check how the workflow could accept a crate parameter and change into the crate root for bellpepper-gadgets and bellpepper. Done means the MSRV lint works for repositories containing multiple crates.

Written by the indexing model from the issue text.