I missed the second-stage OpenSBI job when debugging the v1.  It turns
out that the Docker update dropped python3, so this adds it back in via
apt.

I ran the whole CI this time (via push-ci-now from the docs) and I'm
getting two errors:

    clang-system:
        46/96 qemu:qtest+qtest-arm / qtest-arm/npcm7xx_timer-test           ERROR           1.14s   exit status 1 4321
    
    pages:
        htags: Negative exec line limit = -4825 37

As far as I can tell those aren't failing upstream, but they also don't look
related.  I'm not sure if this has something to do with my fork, I'll try and
poke around a bit but figured I'd send it out in case someone else has seen
something similar.


Changes since v1 <20230224212543.20462-1-palmer@rivosinc.com>:

* Installs python3, which OpenSBI depends on (via kconfiglib).

Link: https://gitlab.com/palmer-dabbelt/qemu/-/pipelines/795696098
Link: https://gitlab.com/qemu-project/qemu/-/pipelines/795635851
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>