Properly handle enviroment variables
This script is growing bigger and bigger and while working on !17 I noticed that the environment variables from the Linux build are causing the Windows build to fail, because
os.environ.copy() doesn't seem to do what it pretends to do. I.e., we still modify the original environment variables after the copy and thus I now manually pop them after each build (15971b47).
Since this is quite ugly, we should do something better in the future.
There are simple helper functions like this one which would allow us to really only temporary set environment variables, but since this would require larger architectural changes, I didn't do it yet. Especially given that we might use official Tor binaries soon anyway.