nixos/activation: fix systemd-user daemon-reload in auto-upgrade serv…

…ice (#47695)

The autoupgrade service defined in `system.autoUpgrade`
(`nixos/modules/installer/tools/auto-upgrade.nix`) doesn't have `su` in
its path and thus yields a warning during the `daemon-reload`.

Specifying the absolute path fixes the issue.

Fixes #47648

nixos/modules/system/activation/switch-to-configuration.pl

@@ -419,7 +419,7 @@ sub filterUnits {
     my ($uid, $name) = ($+{uid}, $+{user});
     print STDERR "reloading user units for $name...\n";
 
-    system("su", "-s", "@shell@", "-l", $name, "-c", "XDG_RUNTIME_DIR=/run/user/$uid @systemd@/bin/systemctl --user daemon-reload");
+    system("@su@", "-s", "@shell@", "-l", $name, "-c", "XDG_RUNTIME_DIR=/run/user/$uid @systemd@/bin/systemctl --user daemon-reload");
 }
 
 close $listActiveUsers;

nixos/modules/system/activation/top-level.nix

@@ -109,6 +109,7 @@ let
     inherit (pkgs) utillinux coreutils;
     systemd = config.systemd.package;
     shell = "${pkgs.bash}/bin/sh";
+    su = "${pkgs.shadow.su}/bin/su";
 
     inherit children;
     kernelParams = config.boot.kernelParams;