seahorse: fix missing icons

Recent `wrapGAppsHook` change stopped `gcr` from being added to `XDG_DATA_DIRS`:
b1e73fa2e0
But gcr installs icons expected by Seahorse to the hicolor theme so we need to add it back.

Also drop adwaita-icon-theme since whether it will be used depends on user’s environment.

pkgs/desktops/gnome/apps/seahorse/default.nix

@@ -51,7 +51,6 @@ stdenv.mkDerivation rec {
     gcr
     gsettings-desktop-schemas
     gnupg
-    gnome.adwaita-icon-theme
     gpgme
     libsecret
     avahi
@@ -78,6 +77,13 @@ stdenv.mkDerivation rec {
     export HOME=$TMPDIR
   '';
 
+  preFixup = ''
+    gappsWrapperArgs+=(
+      # Pick up icons from Gcr
+      --prefix XDG_DATA_DIRS : "${gcr}/share"
+    )
+  '';
+
   passthru = {
     updateScript = gnome.updateScript {
       packageName = pname;