material-maker: fix crash by using export templates from nixpkgs

This was previously using a vendored export template built from godot
3.5.2. This probably happened to work when godot3-headless used the same
version, but now that godot3 is on 3.6, you would get this fatal error
when launching material-maker:

    ERROR: Pack created with a newer version of the engine: 3.6.
       at: try_open_pack (core/io/file_access_pack.cpp:217)

Instead, use the export templates in nixpkgs (godot3-export-templates).
I copied an idiom I saw used for the other godot3 packages.

pkgs/by-name/ma/material-maker/package.nix

@@ -3,6 +3,7 @@
   stdenv,
   fetchFromGitHub,
   godot3-headless,
+  godot3-export-templates,
   libglvnd,
   libX11,
   libXcursor,
@@ -46,6 +47,16 @@ stdenv.mkDerivation (finalAttrs: {
 
     export HOME=$TMPDIR
 
+    # Link the export-templates to the expected location. The --export commands
+    # expects the template-file at .../templates/{godot-version}.stable/linux_x11_64_release
+    mkdir -p $HOME/.local/share/godot
+    ln -s ${godot3-export-templates}/share/godot/templates $HOME/.local/share/godot
+
+    # Don't use the included export template, which might use a mismatched version of godot.
+    rm ./material_maker/misc/linux/godot.x11.opt.64
+    substituteInPlace ./export_presets.cfg \
+      --replace-fail '"material_maker/misc/linux/godot.x11.opt.64"' '""'
+
     mkdir -vp build
     godot3-headless -v --export 'Linux/X11' build/material-maker