2af6e6e7

By: Greg Pfeil <greg@technomadic.org>

Get the ShellCheck module working

There were a bunch of small issues in the way here

- older Nixpkgs conflicted with the `lint-short-path-literals` setting, so we
  had to skip evaluating `gnome2` in those;
- for some reason, the `shellcheck` check would only ever run on one file … and
  a file that only exists in this repo, nothing downstream; and
- there was no way to specify files that ShellCheck should check.

After these changes, this repo correctly inherits the directives from Flaky, and
so we could remove one manually-specified directive from project-manager.bash.

And this enables the ShellCheck module explicitly so that it’s tested here,
rather than simply relying on the Treefmt handling.