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 88.83%. Comparing base (4a13a62) to head (1d39ee1).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61790      +/-   ##
==========================================
- Coverage   89.76%   88.83%   -0.93%     
==========================================
  Files         675      675              
  Lines      204674   204806     +132     
  Branches    39330    39269      -61     
==========================================
- Hits       183716   181946    -1770     
- Misses      13235    15084    +1849     
- Partials     7723     7776      +53     

see 121 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[aduh95]

Suggested change

	--arg ccache '${{ (github.base_ref == 'main' || github.ref_name == 'main') && 'import <nixpkgs> {}).sccache' || 'null' }}' \
	--arg ccache '${{ (github.base_ref == 'main' || github.ref_name == 'main') && '(import <nixpkgs> {}).sccache' || 'null' }}' \

[aduh95]

Can we apply the same here, to avoid repetition