nixos: systemd: add missing sliceToUnit (#179841)

Seem to be omitted in https://github.com/NixOS/nixpkgs/commit/022b4209a398b0648e4d5e2bead42ad9662ad764#diff-5ea4693beb4bdd8f7efcdd3204ceea67c86f59b3d1c28cde063ddf03e1d20fbfL29.

Et7f3 6b820ecf d9d051d6

Changed files
+1
nixos
modules
system
boot
systemd
+1
nixos/modules/system/boot/systemd/user.nix
···
generateUnits
targetToUnit
serviceToUnit
+
sliceToUnit
socketToUnit
timerToUnit
pathToUnit;