rebuild when python interpreter is broken

[ctheune]

I accidentally rsynced an .appenv directory from Mac to Linux and appenv didn't automatically recover from that:

subprocess.CalledProcessError: Command '['./.appenv/unclean/bin/python -m pip install -r requirements.txt --upgrade']' returned non-zero exit status 126.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./batou", line 444, in <module>
    main()
  File "./batou", line 438, in main
    appenv.run(application_name, sys.argv[1:])
  File "./batou", line 234, in run
    self._prepare()
  File "./batou", line 253, in _prepare
    cmd('{env_dir}/bin/python -m pip install -r requirements.txt --upgrade'
  File "./batou", line 43, in cmd
    raise ValueError(e.output.decode("ascii"))
ValueError: /nix/store/9ywr69qi622lrmx5nn88gk8jpmihy0dz-bash-4.4-p23/bin/sh: ./.appenv/unclean/bin/python: cannot execute binary file: Exec format error