Never use tar on windows, and don't use the old-style call() function in new code

commit 79da6077:

cerbero: Fix recipe and bootstrap unpack errors on Windows
This regressed after https://gitlab.freedesktop.org/gstreamer/cerbero/merge_requests/197

tar is much faster than tarfile, but the tar that we use on Windows is
an ancient MSYS tar with many bugs that make it impossible to use it.
So, we use tar only if we're not running on Windows.

commit 9c248d3d:

cerbero: Use the new subprocess API for unpacking
This avoids quoting issues. We should never, ever, use the old call
API for new code. A list of arguments is always going to be correct
and devoid of quoting issues, at least on platforms that natively
support argv[].

Also, pass -C as the first argument to tar because the order can
matter sometimes.

commit 183f895e:

cerbero: Fix circular imports in shell.py
Import cerbero.hacks inside the function call that needs it to avoid
a top-level circular dependency in the importing. This is not
a problem right now, but it's a ticking time-bomb.