fish-shell/fish-shell@3.1.0...3.1.1

The patch we had to use for Apple SDKs was merged upstream, so it can be
dropped. I ran nixpkgs-fmt, and removed the `with stdenv.lib;` scope
expander.

Additionally, did a little bit of cleanup. I plan on refactoring this
more down the line, but this'll do for now.

I finally figured out why we use `fetchurl` for the tagged release: the
published release tarballs contain a version file, which the
`build_tools/git_version_gen.sh` script reads (and uses as the version
if it exists). The other thing it contains are pre-generated docs for
various `fish` builtins. I've expanded the comment to document this so
nobody is as confused as I was when I first saw it. (Though I plan to
change this and add sphinx as a native build input in order to build the
docs ourselves.)

We don't have access to the codesign binary.

fish: 3.1.0 -> 3.1.1