On ostree, fish has a configuration directory at /etc/fish/ but its fish is not compiled to read from it -- probably due to a packaging bug:

foo@fedora ~> echo $__fish_vendor_confdirs
/var/home/foo/.local/share/fish/vendor_conf.d /var/home/foo/.local/share/flatpak/exports/share/fish/vendor_conf.d /var/lib/flatpak/exports/share/fish/vendor_conf.d /usr/local/share/fish/vendor_conf.d /usr/share/fish/vendor_conf.d

It is compiled to read from /usr/local/share/fish, but that directory doesn't exist, so we don't choose to write to it.

If we detect the host has fish shell, we should do the extra work to detect if we're on an ostree-based distro and do the right thing. I think this means we create /usr/local/share/fish and write our nix.fish file to /usr/local/share/fish/vendor_conf.d/.

This will resolve our highest source of installation errors (SelfTest("ShellFailed("fish")").