ci: Fix path filter in PR Reporter action

[rschristian]

Follow up to #4643

The action is running again but it falls over as it cannot use the branch name from the fork as a reference to compare against -- that'll only work for local branches. To fix this, we simply switch over to the SHA, should do the trick for both forks & local branches all the same.

Tests here, if anyone cares to take a look. Top 3 are a local branch (real workflow + 2 workflow_run-triggered events), 3 after that are from a fork.

[coveralls]

coverage: 99.618%. remained the same
when pulling f1184bf on rschristian-patch-1
into 818eafb on main.

[rschristian]

Posting this here for myself in the future: whilst it seems like an obvious improvement to copy this to the CI workflow, ensuring it filters based on the latest commit only (thereby skipping running the benches for commits that haven't changed the source JS themselves), unfortunately dorny/paths-filter does not allow configuring base & ref parameters for PRs for some reason.

That might (& hopefully will) change, but for now, this is the best we can do short of splitting off the benches too, which I don't find particularly compelling.