Add cargo-shear to lint checks

[Mshehu5]

Detect unused Rust dependencies with cargo-shear.
The check runs as part of the Nix lint suite and the tool is available in the development shell for local use.

[nothingmuch]

utACK.

validate commits has an EXPECT-FAIL thing you can use to validate that this work: make a new commit with [EXPECT-FAIL: cargo-shear] in the title, cargo add an unnecessary dep, and validate-commits will ensure that the corresponding flake check fails.

i don't yet have infrastructure for this, the plan was to have these as commits that branch off the commit that introduces the flake check, and then maintain a "check-fixtures" branch or something like that which octo-merges those. then in the CI job, the octo merge can be used to find those commits, and each could be rebased on top of GITHUB_REF to ensure that the flake check has not regressed. for now i have been testing these locally where i've added them.

feel free to make this into a new issue if interested

[bc1cindy]

utACK f12ae70