Manually trigger optional E2E tests

### What is this about?

Recently a number of E2E test jobs were made optional on PRs. They are now only triggered on long-running branches.

We should add a way to manually trigger these jobs, in case a PR is expected to have some impact upon them. For example, if we fix a test related to Firefox, we should run the Firefox E2E tests before merging.

### Scenario

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### Design

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### Technical Details

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### Threat Modeling Framework

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### Acceptance Criteria

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### Stakeholder review needed before the work gets merged

- [X] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

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Start by locating the CI definitions for the optional E2E jobs that run on long-running branches and reviewing how pull-request triggers are configured. Add a manual way to run the relevant jobs, including the Firefox E2E tests, and verify that a pull request can invoke them before merging.

Written by the indexing model from the issue text.