hooks: add moveSystemdUserUnitsHook

This hook moves systemd user service file from `lib/systemd/user` to
`share/systemd/user`. This is to allow systemd to find the user
services when installed into a user profile. The `lib/systemd/user`
path does not work since `lib` is not in `XDG_DATA_DIRS`.

pkgs/stdenv/generic/default.nix

@@ -61,7 +61,10 @@ let
     ]
       # FIXME this on Darwin; see
       # https://github.com/NixOS/nixpkgs/commit/94d164dd7#commitcomment-22030369
-    ++ lib.optional hostPlatform.isLinux ../../build-support/setup-hooks/audit-tmpdir.sh
+    ++ lib.optionals hostPlatform.isLinux [
+      ../../build-support/setup-hooks/audit-tmpdir.sh
+      ../../build-support/setup-hooks/move-systemd-user-units.sh
+    ]
     ++ [
       ../../build-support/setup-hooks/multiple-outputs.sh
       ../../build-support/setup-hooks/move-sbin.sh