make mutants job diff aware

[nothingmuch]

https://mutants.rs/pr-diff.html

in the context of validate-commits and nix flake check passing for every commit in main, it makes sense to define the flake mutants such that it work --in-diff for the corresponding commit diff

[nothingmuch]

the cleanest way to do this is to have validate-commits add diffs for the commit range to the storage pah, and have the flake define an input:

inputs.mutants-diff = {
  url = "path:/dev/null";
  flake = false;
};

that validate-commits can then --override-input after using nix store add-path to insert the diff for the specified commit

[nothingmuch]

if we make the mutants check dependent nextest check, we can enable --baseline=skip https://mutants.rs/baseline.html