flake: let module callers supply pkgs so overlays reach the build (#2244)

The nixosModule/homeModule path previously called `buildDmsPkgs pkgs` but
internally referenced `self.packages.${system}.default`, which was
instantiated via `nixpkgs.legacyPackages`, an unoverlayed pkgs. That
meant downstream flakes couldn't reach through their own overlays to
the dms-shell build (e.g. to swap `kdePackages.sonnet` or trim perl
out of the aspell closure).

Extract the derivation as `mkDmsShell = pkgs: ...` at the top-level
`let`, and call it from both `packages.${system}.dms-shell` (for
direct consumers of the flake) and `buildDmsPkgs pkgs` (for module
consumers, which now pass in the system's overlayed pkgs).

Also re-checks overrideAttrs / .override still work: `mkDmsShell pkgs`
is the same `pkgs.lib.makeOverridable` wrapper as before, just
parameterized on the caller's pkgs instance.

Co-authored-by: Lucas <43530291+LuckShiba@users.noreply.github.com>

flake.nix

@@ -50,10 +50,7 @@
         nixpkgs.lib.genAttrs [ "aarch64-linux" "x86_64-linux" ] (
           system: fn system nixpkgs.legacyPackages.${system}
         );
-      buildDmsPkgs = pkgs: {
-        dms-shell = self.packages.${pkgs.stdenv.hostPlatform.system}.default;
-        quickshell = quickshell.packages.${pkgs.stdenv.hostPlatform.system}.default;
-      };
+
       mkModuleWithDmsPkgs =
         modulePath:
         args@{ pkgs, ... }:
@@ -62,6 +59,7 @@
             (import modulePath (args // { dmsPkgs = buildDmsPkgs pkgs; }))
           ];
         };
+
       mkQmlImportPath =
         pkgs: qmlPkgs:
         pkgs.lib.concatStringsSep ":" (map (o: "${o}/${pkgs.qt6.qtbase.qtQmlPrefix}") qmlPkgs);
@@ -78,10 +76,11 @@
           qtimageformats
           kimageformats
         ];
-    in
-    {
-      packages = forEachSystem (
-        system: pkgs:
+
+      # Allows downstream modules to provide their own 'pkgs' (with overlays)
+      # instead of being forced to use the flake's locked nixpkgs.
+      mkDmsShell =
+        pkgs:
         let
           mkDate =
             longDate:
@@ -99,8 +98,7 @@
             in
             "${cleanVersion}${dateSuffix}${revSuffix}";
         in
-        {
-          dms-shell = pkgs.lib.makeOverridable (
+        pkgs.lib.makeOverridable (
           {
             extraQtPackages ? [ ],
           }:
@@ -180,8 +178,16 @@
           )
         ) { };
 
+      buildDmsPkgs = pkgs: {
+        dms-shell = mkDmsShell pkgs;
+        quickshell = quickshell.packages.${pkgs.stdenv.hostPlatform.system}.default;
+      };
+    in
+    {
+      packages = forEachSystem (
+        system: pkgs: {
+          dms-shell = mkDmsShell pkgs;
           quickshell = quickshell.packages.${system}.default;
-
           default = self.packages.${system}.dms-shell;
         }
       );