build-qemu-robot-docker: Use pip mirror

When PIP_MIRROR env is set, setup the pip mirror in $HOME/.pip/pip.conf
to speedup the pip download.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I4bac9faafbad375772f911eb0675522215f62405
1 file changed