fontforge: Fix uninterpolated `CMAKE_INSTALL_PREFIX` in RPATH.
fontforge: Fix uninterpolated `CMAKE_INSTALL_PREFIX` in RPATH.

This was introduced in https://github.com/NixOS/nixpkgs/pull/89583
and fixed upstream with a master-only patch in
https://github.com/fontforge/fontforge/pull/4232.

Found via
https://github.com/nh2/static-haskell-nix/pull/98#issuecomment-665395399.

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