GitHub actions simplified -- should be easier to find build status now

[Ash Berlin-Taylor]

Hi all,

I've just merged <https://github.com/apache/airflow/pull/15944> which 
makes some changes to our Ci pipelines on GitHub actions to use 
new(ish) features.

Primarily this means that the Build Image jobs will now show up 
directly in the "Checks" of the PR, making it much more obvious when 
they fail.


See the PR for more details.

If anyone notices anything odd going on with CI ping me here or in 
#development on Slack

-ash