Enable pushing early image cache to ghcr.io registry

[potiuk]

In order to react quicker to changes in setup.py, setup.cfg and
Dockerfiles, we can push an early image cache to the ghcr.io cache.

This build is done way before constraints are regenerated so it is
using the "current" constraints at the moment the merge happens.
This should be safe - if the image fails to build (we ignore that)
the cache will not be pushed, but if it will, then even if we merge
a new commit quickly and the rest of the build is cancelled, the
cache will be refreshed with new Dockerfile/setup.py/setup.cfg and
the subsequent builds will run much faster.

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[potiuk]

This is a reasult of discussion with @o-nikolas and @mik-laj on Slack: https://apache-airflow.slack.com/archives/C015SLQF059/p1661905832810429

This change should make our cache of images much "fresher" even in case our main starts failing. It does not attempt to upgrade constraints - it will try to build the image with current constraints and push it as cache. This is done early in the build workflow and it will not wait for all tests to finish (like the final cache pushing which also upgrades constraints).

Result of this change will be that even if tests fail, if the images can be build and succeed, they will be pushed as cache quickly - and this will make subsequent builds to be much faster.

[o-nikolas]

Thanks for the quick turnaround!