ci: split build workflow

[hoxyq]

Summary

We can parallelise debug and release builds and save ~15 minutes for every PR.

Test plan

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.

[huntie]

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