Post-Build Step is broken on PRs

Stream: mathlib4

[Ruben Van de Velde]

It turns out the Post-Build Step is just running on master rather than on the PR branch (documented here)

[Kim Morrison]

It does appear to be incorrect. :-(

#26205 breaks undergrad.yaml, but CI accepts it:

https://github.com/leanprover-community/mathlib4/actions/runs/15777154991/job/44474399999?pr=26205#step:6:43

[Kim Morrison]

#26207

[Kim Morrison]

I've verified that #26205 now fails, post #26207, i.e. the post-build steps are running on the PR.