Set allow-failure on every try scenario

[NullVoxPopuli-ai-agent]

Summary

Fixes the silent failure on the Vite branch where the try-scenarios job never appears in the CI run but the overall run is marked failure.

The workflow has:

try-scenarios:
  continue-on-error: ${{ matrix.allow-failure }}
  ...
  strategy:
    fail-fast: false
    matrix: ${{fromJson(needs.build_matrix.outputs.matrix)}}

@embroider/try's list command passes scenario objects through almost verbatim (only env: {} is defaulted in). Since none of the scenarios in test-app/.try.mjs defined allow-failure, ${{ matrix.allow-failure }} evaluated to '', which GitHub Actions rejects as a non-boolean. The annotation on run 24614721563 confirms:

Error when evaluating 'continue-on-error' for job 'try-scenarios'. .github/workflows/ci.yml (Line: 170, Col: 24): Unexpected value ''

Because the matrix failed to materialize, no try-scenarios job was created, yet the workflow run was marked failure — which is why the 13 visible checks are all green while the PR shows red.

This PR defines allow-failure on every scenario: true for the bleeding-edge ember-beta and ember-alpha scenarios, false everywhere else.

Test plan

• CI on this PR shows the try-scenarios jobs (min-supported, ember-lts-4.12, ember-lts-5.12, ember-lts-6.4, ember-latest, ember-beta, ember-alpha) actually get created
• Annotation about continue-on-error evaluating to '' no longer appears
• Overall workflow conclusion matches the visible check results

🤖 Generated with Claude Code