libfx2: fix git ref
libfx2: fix git ref

cache.nixos.org has the actual source cached, so this didn't impact
builds for anyone using that cache.

    edef@jaguar ~/s/nixpkgs> nix eval -I nixpkgs=. nixpkgs.pythonPackages.fx2.src.urls
    [ "https://github.com/whitequark/libfx2/archive/0.9.tar.gz" ]
    edef@jaguar ~/s/nixpkgs> curl -L https://github.com/whitequark/libfx2/archive/0.9.tar.gz
    404: Not Found

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