nixos/systemd: Handle template overrides

Adding template overrides allows for custom behavior for specific
instances of a template. Previously, it was not possible to provide
bind mounts for systemd-nspawn. This change allows it.

nixos/modules/system/boot/systemd-lib.nix

@@ -182,7 +182,18 @@ in rec {
       # upstream unit.
       for i in ${toString (mapAttrsToList (n: v: v.unit) units)}; do
         fn=$(basename $i/*)
-        if [ -e $out/$fn ]; then
+
+        case $fn in
+          # if file name is a template specialization, use the template's name
+          *@?*.service)
+            # remove @foo.service and replace it with @.service
+            ofn="''${fn%@*.service}@.service"
+            ;;
+          *)
+            ofn="$fn"
+        esac
+
+        if [ -e $out/$ofn ]; then
           if [ "$(readlink -f $i/$fn)" = /dev/null ]; then
             ln -sfn /dev/null $out/$fn
           else