Download using powershell on Windows and rework download func

cerbero gen-cache was hanging on the MSVC post-merge CI intermittently for some time, and recently it has started hanging consistently. Debugging on the runner revealed that urllib2 was stuck somewhere that could not be determined while downloading the artifact deps log, and that it wasn't reading or writing anything. Most recently:

https://gitlab.freedesktop.org/gstreamer/cerbero/-/jobs/28839005

urllib2 has always been slow and buggy, so let's just get rid of it entirely in favour of Invoke-WebRequest in powershell.

The only side-effect of this is that we now require curl or wget on non-Windows platforms, which is not a problem.