Test flakiness in CI per concurrent workflow runs and/or auth issues

[mnholtz]

Follow-up of https://github.com/pixiebrix/pixiebrix-extension/issues/9087

We're seeing intermittent flakiness in CI due to either

1. Concurrent workflows being run (by e.g. dependabot PRs being open all at once)
2. and/or the auth setup failing (of the past workflow failures due to e2e tests in the past few hours, e.g. here, here, here, and here) all but one were failing due to auth setup issues

We still need to investigate to identify root cause.