Allow individual variant builds to fail without cancelling others

[ayoy]

Task/Issue URL: https://app.asana.com/0/1203301625297703/1206145260961053/f

Description:
Disable fail-fast flag for the variants matrix so that a single failed job won't cause others to get cancelled.

Steps to test this PR:

1. Check out this workflow run and verify that 1 of 10 jobs has failed, but others have successfully completed.
2. Check out the re-run and verify that all jobs are successful.
3. Check out the usage for that workflow and verify that only the lc job was billed (run) twice.

---

Internal references:

Pull Request Review Checklist
Software Engineering Expectations
Technical Design Template
Pull Request Documentation

[samsymons]

Test steps look great, and the billing page checks out as well. 👍 Thanks for this one! It should make the Monday release process even smoother.