sigrok-cross-mingw: Temporarily unbreak Windows CI builds in dirty ways.

A typical use of the cross-compile scripts are CI builds, which currently
fail in the Python tarball download step. Disable certificate checks when
sigrok.org build machines fetch data from the sigrok.org site. Keep the
Zadig download unaffected.

Another typical use is the convenient local compilation from source at
users' sites, which should be rare since nightlies are available, while
developers don't run the cross-compile scripts which are designed for
single shots instead of multiple iterations including local source code
modifications. Which should make this change acceptable for this use
case as well, and happens to unbreak it, too.

Add a TODO comment, ideally this commit soon gets reverted.