CI: `smoke.yaml` runs tests on main branch instead of pr branch when triggered by pr

[fynnsu]

Problem line is:

training/.github/workflows/smoke.yaml

Line 130 in c1c1c45

ref: ${{inputs.branch}}

Note: this works as expected when called with workflow_dispatch but fails when triggered by pull_request_target trigger.

Example run checking out incorrect branch: https://github.com/instructlab/training/actions/runs/14886706608/job/41808924082#step:6:177

[fynnsu]

Closing now that #549 is merged