cabal support #239

build attempt 1...
    => error
    [INFO] Installing environment for https://github.com/NixOS/nixfmt.
    [INFO] Once installed this environment will be reused.
    [INFO] This may take a few minutes...
    An unexpected error has occurred: CalledProcessError: command: ('cabal', 'update')
    return code: 1
    stdout:
        Executable `cabal` not found
    stderr: (none)
    Check the log at /pc/pre-commit.log