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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ERROR: Pack created with a newer version of the engine: 3.6.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.