Fix BCApps e2e: scope PR Build run lookup to e2etest branch (#2329)

### ❔What, Why & How

The `BCApps` e2e scenario intermittently fails during the Pull Request
Build wait with a `404 (Not Found)` thrown from `WaitWorkflow`.

Root cause: the run lookup in `e2eTests/scenarios/BCApps/runtest.ps1`
filtered `/actions/runs` only by `event -eq 'pull_request'` and `name
-eq 'Pull Request Build'`. When a concurrent Dependabot PR is open in
the temp repo, its build run is also named "Pull Request Build", so the
filter returns more than one run and `$run` becomes an array.
`WaitWorkflow`'s `[string] $runid` parameter then coerces that array to
a space-joined string (`"<id1> <id2>"`), producing an invalid
`/actions/runs/<id1> <id2>` URL that 404s.

The longer runtime introduced by #2306 (pushing the `projects` change to
`main` and waiting for a full CI/CD build) widened the window for
Dependabot to open its PR before the `e2etest` PR is created, which is
why this surfaced now. The underlying filter bug predates that change.

Fix: scope the lookup to `head_branch -eq $branch` (the `e2etest`
branch) and take the most recent match with `Select-Object -First 1`, so
`$run.id` is always a single value.

### ✅ Checklist

- [x] Add tests (E2E, unit tests)
- [ ] Update RELEASENOTES.md
- [ ] Update documentation (e.g. for new settings or scenarios)
- [ ] Add telemetry

N/A for RELEASENOTES / documentation / telemetry: this only touches
internal e2e test infrastructure and is not user facing.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>