Motivation for this change

Primarily need to specify where to install bash completions,
or the build fails attempting to write to invalid location.

The rest is because I was going cross-eyed checking I didn't typo
one of the "dir" flags, but I can see this either way.

General trend seems to be away from "$(out)" usage here,
in favor of the placholder builtin for the
definitely-not-shell-escape-problem factor :).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.