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 4 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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[aduh95]

Can we avoid the repetition?

Suggested change

	CC: ${{ (github.base_ref == 'main' || github.ref_name == 'main') && 'sccache clang-19' || 'clang-19' }}
	CXX: ${{ (github.base_ref == 'main' || github.ref_name == 'main') && 'sccache clang++-19' || 'clang++-19' }}
	CC: ${{ (github.base_ref == 'main' || github.ref_name == 'main') && 'sccache' || '' }} clang-19
	CXX: ${{ (github.base_ref == 'main' || github.ref_name == 'main') && 'sccache' || '' }} clang++-19

[aduh95]

We should set it to null otherwise it defaults to ccache (which would be pure overhead on a ephemeral runner)

Suggested change

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

Alternatively, we could define it in the Configure sccache step above, and fallback to null if it was skipped

Suggested change

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

[aduh95]

nit: I would simplify this, it's no big deal if we pass not-defined variables

Suggested change

	${{ (github.base_ref == 'main' || github.ref_name == 'main') && '--keep SCCACHE_GHA_VERSION --keep ACTIONS_CACHE_SERVICE_V2 --keep ACTIONS_RESULTS_URL --keep ACTIONS_RUNTIME_TOKEN' || '' }} \
	--keep SCCACHE_GHA_VERSION --keep ACTIONS_CACHE_SERVICE_V2 --keep ACTIONS_RESULTS_URL --keep ACTIONS_RUNTIME_TOKEN \