* workflows/check-nix-format: Enforce nixfmt on new/changed files
This makes the Nix format workflow check new/changed files instead of
just an allowlist.
This enforces that all PRs updated after this is merged are required to
have fully standard formatted Nix files!
* workflows/check-nix-format: determine changed files via base commit
The next commit will use this to have a simpler change
* workflows/check-nix-format: Only ensure for already formatted files
This prevents situations where contributors need to suddenly format a
huge file even if they only changed a small part of it (e.g.
all-packages.nix)
