nixos/systemd-lib: fix conflict with dbus.service.d directory

When a package contains a directory in one of the systemd directories
(like flatpak does), it is symlinked into the *-units derivation.
Then later, the derivation will try to create the directory, which
will fail:

mkdir: cannot create directory '/nix/store/…-user-units/dbus.service.d': File exists
builder for '/nix/store/…-user-units.drv' failed with exit code 1

Closes: #33233