mkvenv: work around broken pip installations on Debian 10

mkvenv: work around broken pip installations on Debian 10

This is a workaround intended for Debian 10, where the debian-patched
pip does not function correctly if accessed from within a virtual
environment.

We don't support Debian 10 as a build platform any longer, though we do
still utilize it for our build-tricore-softmmu CI test. It's also
possible that this bug might appear on other derivative platforms and
this workaround may prove useful.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-11-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>