gtklock-powerbar-module: Fix systemctl command paths

Replace hardcoded "systemctl" commands with full store paths to ensure
the module works correctly in NixOS environments.

pkgs/by-name/gt/gtklock-powerbar-module/package.nix

@@ -7,6 +7,7 @@
   pkg-config,
   gtk3,
   gtklock,
+  systemd,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -28,6 +29,17 @@ stdenv.mkDerivation (finalAttrs: {
 
   buildInputs = [ gtk3 ];
 
+  postPatch =
+    let
+      systemctl = lib.getExe' systemd "systemctl";
+    in
+    ''
+      substituteInPlace source.c \
+        --replace-fail '"systemctl reboot"' '"${systemctl} reboot"' \
+        --replace-fail '"systemctl -i poweroff"' '"${systemctl} -i poweroff"' \
+        --replace-fail '"systemctl suspend"' '"${systemctl} suspend"'
+    '';
+
   passthru.tests.testModule = gtklock.testModule finalAttrs.finalPackage;
 
   meta = {