Update GitHub Actions workflows. (#728)

This PR was automatically generated by the
update-workflows-ecosystem-providers workflow in the pulumi/ci-mgmt
repo, from commit 06a4a07bc11ada9498ce86091ac0d55443ee294b.

Author: pulumi-bot

.github/workflows/run-acceptance-tests.yml

@@ -44,7 +44,7 @@ jobs:
     uses: ./.github/workflows/prerequisites.yml
     secrets: inherit
     with:
-      default_branch: ${{ github.event.repository.default_branch }}
+      default_branch: ${{ github.event.pull_request.base.ref }}
       is_pr: ${{ github.event_name == 'pull_request' }}
       is_automated: ${{ github.actor == 'dependabot[bot]' }}