Build CI images for the merge result of a PR, not the tip of the PR (#18060)
Build CI images for the merge result of a PR, not the tip of the PR (#18060)

The change to use pull_request_target had the unintended side-effect of
meaning that the images were built using the tip of the PR, instead of
the merge of main and the PR branch.

This restores that behaviour (and should mean that fewer PRs will need
rebasing to fix their builds when master gets broken)
bolkedebruin/airflow,danielvdende/incubator-airflow,apache/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,apache/airflow,bolkedebruin/airflow,apache/airflow,lyft/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,apache/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,apache/airflow,bolkedebruin/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,lyft/incubator-airflow,apache/airflow,apache/airflow,lyft/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,apache/airflow