perf(ci): only save ccache on main branch

[mikehardy]

Description

prevents LRU eviction thrashing of caches since we are near the limit on useful caches for a single CI run

This is a follow-on to the CI perf tuning PR from a few days back, it was the only thing I noticed that wasn't perfect after merging that one - we are saving lots of ccache's, mostly from branches, and it LRUs out useful main-branch caches

Related issues

• test(ci): implement cache best practices for github workflows #8820

Release Summary

test only, and just perf on test even - no release

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
  • Other (macOS, web)
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

Look at the caches page after this is implemented, and we should no longer see caches from non-branch that start with ccache https://github.com/invertase/react-native-firebase/actions/caches

Additionally, after other branches update to this workflow or run them and it has been a while, we will hopefully stop seeing our cacches expire so frequently - many of them should be pretty long-lived (weeks+, not just hours/days)

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Project	Deployment	Review	Updated (UTC)
react-native-firebase	Ready	Preview, Comment	Jan 5, 2026 5:03pm

[mikehardy]

Not saving cache because 'save' is set to 'false'.

worked (example https://github.com/invertase/react-native-firebase/actions/runs/20722896996/job/59491170948?pr=8822#step:75:28)

will verify main saves post-merge