Dockerfile.build-debian-mips: use buster with libunwind fix

We can now use libunwind 1.2.1-10~deb10u1 from Debian Buster as per
this bug report which has now been resolved:

  https://bugs.debian.org/932725



Drop stretch-backports for MIPS and use Buster like the other
architectures.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>