- Format using nixfmt
- Apply other minor formatting tweaks
- Simplify `--filter` test flag definition

We still need to fix the tests to use `finalAttrs`, but that requires
changes to `buildDotnetModule`.

Use `override` to pass a `_7zz` with RAR support to the package, instead
of having an `enableUnfree` package argument.

Use a different `pname` on the unfree package.

Use `_7zz.meta.unfree` internally.

Add a long description that documents whether or not RAR archive mods
are supported and how to enable support.