[CI] Difference between premerge and postcommit tests

[boomanaiden154]

https://lab.llvm.org/staging/#/builders/192/builds/1361 failed postcommit but the premerge checks in llvm/llvm-project#147787 appear to have passed.

[joker-eph]

This is a mid-air collision: the test that failed post-merge didn't exist when the PR was created.
Test added in 8b64cd8be29da9ea74db5a1a21f7cd6e75f9e9d8
PR landed in 13eca5248c7bf625af9c7af898d48e8c0a441496

[boomanaiden154]

Ah, thanks for investigating. That's a much harder problem to fix then.

I guess +1 for merge queues.