setupSystemdUnits: use /nix/var/nix/profiles/default/lib/systemd/system as fallback

The previous commit stopped systemd from looking for system units in
/etc/systemd-mutable/system, which was a Dysnomia-specific path.

While this script doesn't seem to be used anywhere inside nixpkgs (also
not in the gone-since #110799 Dysnomia), its fallback mode (when
/etc/systemd/system is read-only) did write units to that
Dysnomia-specific path, which systemd now doesn't look at anymore.

It might be up for another debate on whether systems with read-only
/etc/systemd/system should probably just use /run/systemd/system, and
not some NixOS-specific paths, as such conditions can happen on other
distros too, but let's pick the other NixOS-specific path
/nix/var/nix/profiles/default/lib/systemd/system for now, which is
probably better than a path that surely is never looked at.

pkgs/build-support/setup-systemd-units.nix

···
 
           unitDir=/etc/systemd/system
           if [ ! -w "$unitDir" ]; then
-            unitDir=/etc/systemd-mutable/system
+            unitDir=/nix/var/nix/profiles/default/lib/systemd/system
             mkdir -p "$unitDir"
           fi
           declare -a unitsToStop unitsToStart