nix: fix build by deferring submodule fetching

Currently, it isn't possible to fetch submodules on `inputs.self` of a
Nix flake. As a workaround, use `builtins.fetchGit` with `self.rev` of
the current checkout to include submodules.

This implementation defers submodule fetching to build time instead of
before the flake evaluation begins, which would be the case if it were
possible to specify submodule fetching in the `inputs` of `self` as with
other inputs. This way, when interacting with the other outputs of the
flake, the cost of fetching submodules is avoided.

nix/default.nix

@@ -1,4 +1,5 @@
 {
+  self,
   lib,
   stdenv,
   pkg-config,
@@ -68,7 +69,16 @@ assert lib.assertMsg (!hidpiXWayland) "The option `hidpiXWayland` has been remov
         baseName = baseNameOf (toString name);
       in
         ! (lib.hasSuffix ".nix" baseName);
-      src = lib.cleanSource ../.;
+      src = lib.cleanSource (
+        if self ? rev
+        then builtins.fetchGit {
+          url = "https://github.com/hyprwm/Hyprland";
+          inherit (self) rev;
+          submodules = true;
+          allRefs = true;
+        }
+        else ../.
+      );
     };
 
     postPatch = ''

nix/overlays.nix

@@ -36,6 +36,7 @@ in {
         version = "${props.version}+date=${date}_${self.shortRev or "dirty"}";
         commit = self.rev or "";
         inherit date;
+        inherit (inputs) self;
       };
       hyprland-unwrapped = final.hyprland.override {wrapRuntimeDeps = false;};
       hyprland-debug = final.hyprland.override {debug = true;};