firefox/wrapper: inject gsettings schema paths into XDG_DATA_DIRS (#546288)

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -6,6 +6,7 @@
   lndir,
   config,
   buildPackages,
+  gsettings-desktop-schemas,
   jq,
   xdg-utils,
   writeText,
@@ -134,6 +135,12 @@ let
         ++ pkcs11Modules
         ++ lib.optionals (!isDarwin) gtk_modules;
       gtk_modules = lib.optionals (!isDarwin) [ libcanberra-gtk3 ];
+      # strictDeps prevents buildInputs from populating GSETTINGS_SCHEMAS_PATH.
+      # Revert when https://github.com/NixOS/nixpkgs/pull/546281 hits stable.
+      gsettingsSchemaPaths = lib.optionals (!isDarwin) [
+        "${gsettings-desktop-schemas}/share/gsettings-schemas/${gsettings-desktop-schemas.name}"
+        "${browser.gtk3}/share/gsettings-schemas/${browser.gtk3.name}"
+      ];
 
       # Darwin does not rename bundled binaries
       launcherName = "${applicationName}${lib.optionalString (!isDarwin) nameSuffix}";
@@ -339,6 +346,11 @@ let
         ":"
         "${adwaita-icon-theme}/share"
 
+        "--prefix"
+        "XDG_DATA_DIRS"
+        ":"
+        (lib.concatStringsSep ":" gsettingsSchemaPaths)
+
         "--set-default"
         "MOZ_ENABLE_WAYLAND"
         "1"
@@ -486,9 +498,6 @@ let
             mv "$executablePath" "$oldExe"
           fi
         ''
-        + lib.optionalString (!isDarwin) ''
-          appendToVar makeWrapperArgs --prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH"
-        ''
         + ''
           concatTo makeWrapperArgs oldWrapperArgs