[AIRFLOW-6969] Move backport build to test

[potiuk]

When one of the test builds fails even restarting it does not help to succeed
the whole job if there is "post-test" stage. The post-test job will remain in
cancelled state after the job is restarted

We should move prepare-backport-packages to test phase (it is very fast).

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[mik-laj]

I'm not sure. Maybe we should run it only on master or manually?

[potiuk]

I prefer to run everything, always. This way you find problems immediately when they appear and not after they are merged. I want this to be checked the same way as we check if all tests are running. And it is many times faster than the tests..