[FIX] Fix post_(un)install...

__NB__ The `cwd` directory ["is not considered when searching the executable,
so you can’t specify the program’s path relative to `cwd`."][cwd].  Therefore,
we manually change to the directory containing the hook script and change back
to the original working directory when we're done.

[cwd]: https://docs.python.org/2/library/subprocess.html#popen-constructor