E2E Tests should Block PR Merge

Update GitHub settings for BDT Project to prevent the merging of PRs that cause the E2E test suite to fail.

Link to an example run of the E2E Check:
https://github.com/CodeForPhilly/benefit-decision-toolkit/actions/runs/20731228718/job/59519358200

Link to the file defining the GitHub Workflow:
https://github.com/CodeForPhilly/benefit-decision-toolkit/blob/main/.github/workflows/run-e2e-tests.yml