Eliminate --host_force_python=PY2

[celskeggs]

We'd really like to be using Python 3, but we're stuck on 2 for our host binaries as a result of containerregistry being broken with python 3.

We should eliminate the --host_force_python=PY2 flag from our bazel builds, which will be added in #478.

[cryslith]

containerregistry is no longer broken with python 3. There's also an interesting sentence on the fix PR:

Ideally most folks would migrate over to go-containerregistry, but I understand that rewriting all the python code in the world is impractical.

This is the first I've heard of this other project - I don't recall what our dependency chain to containerregistry looks like but we might want to use that instead if possible.