Fix ReleaseBranches e2e: point previous-release assertions at the new step (#2252)

The `ReleaseBranches` scenario started failing after commit 12b6760
introduced a dedicated `DownloadPreviousRelease` action. The log
messages the test searches for (`No previous release found` and `Using
vX (tag X.Y.Z) as previous release`) used to be emitted from the `Build`
step, but now come from the new `Download Previous Release` step. The
test wasn't updated, so all six `Test-LogContainsFromRun` calls in
`e2eTests/scenarios/ReleaseBranches/runtest.ps1` were looking at the
wrong step and failing.

This updates those six assertions to use `-stepName 'Download Previous
Release'`. No product code changes; test-only fix.

Failing run for reference:
https://github.com/microsoft/AL-Go/actions/runs/25755372648/job/75749344161

Co-authored-by: aholstrup1 <aholstrup1@users.noreply.github.com>