build: skip sscache action on non-main branches

[joyeecheung]

To reduce cache thrashing.

From a look at the action results, PRs targeting non-main generally don't have a good cache hit rate anyway, so it seems better to just keep it to main where the majority of the volume is:

https://github.com/nodejs/node/actions/runs/21866462915
https://github.com/nodejs/node/actions/runs/21866461713

Refs: #61436

[nodejs-github-bot]

Review requested:

• @nodejs/actions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.87%. Comparing base (4a13a62) to head (3698f29).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61790      +/-   ##
==========================================
- Coverage   89.76%   88.87%   -0.89%     
==========================================
  Files         675      675              
  Lines      204674   204674              
  Branches    39330    39245      -85     
==========================================
- Hits       183716   181901    -1815     
- Misses      13235    14994    +1759     
- Partials     7723     7779      +56     

see 99 files with indirect coverage changes

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