[CI] Run postcommit for pulldown PRs (#22627)
We need to run postcommit in pulldown PRs.
In this PR the job is running twice, once for the `push` action and once
for `pull_request` since we modified the postcommit workflow, but in a
normal pulldown won't happen.
We can't use the `pull_request` action because GitHub requires both
`branches` and `paths` to be satisfied which won't be true for a normal
pulldown.
See https://github.com/intel/llvm/pull/22620
Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
