Python dtests upgrade tests mixed SHA runs

Hi adding a bug I noticed in jenkins:
A- Commit SHA1
B- Jenkins starts a test run and eventually starts python dtests
C- Before B completes commit SHA2 comes in

Python dtests in B will run mixed SHA1 and SHA2 versions of trunk. An example https://ci-cassandra.apache.org/job/Cassandra-trunk-dtest-upgrade/1213/jdk=jdk_11_latest,label=cassandra-dtest,split=11/consoleFull where both 2a458ba01dae3b35b62ad2ef361e1acb567c97f0 and the later a565711056c859398d0b26081b46e71d2076de1d get mixed up.