Update E2E tests to run on PRs/branches

[rwood-moz]

After #480 lands, the TB Accounts E2E tests (currently just the one test now, 'submit a request') will run nightly on production as well as on stage after a merge to main/stage deployment. Enable the tests to also run against PRs/branches by updating the validate GHA workflow accordingly.

[MelissaAutumn]

Note I tried to enable it, but it seems like we need to also update the ci with additional env/secrets.
https://github.com/thunderbird/thunderbird-accounts/actions/runs/21227379654/job/61077816689?pr=525

[rwood-moz]

Note I tried to enable it, but it seems like we need to also update the ci with additional env/secrets. https://github.com/thunderbird/thunderbird-accounts/actions/runs/21227379654/job/61077816689?pr=525

Yeah makes sense, I'll take a look.