daemon/setup: expose zccache store location so consumer workflows can actions/cache it

[zackees]

Problem

fbuild's setup composite action installs and configures zccache as the per-TU object cache for Rust builds (and via wrap_args, potentially for compiler invocations). Where zccache persists its object store (~/.zccache/, $ZCCACHE_DIR, or configured override) is not a documented output of the action, so consumer workflows (bench/fastled-examples, FastLED CI) don't know what path to feed actions/cache.

Iter3 of the #112 benchmark confirmed this is load-bearing: 65 MB of .build/pio/uno/.fbuild/ was cached and restored, but cross-run per-TU reuse was zero. Even once #146 + #148 (absolute paths) land, zccache's store is the directory that actually holds compiled .o files — if it's not cached, every cross-run compile re-runs the compiler.

Sub-task of #147.

Fix direction

1. The setup composite action emits a step output zccache-store-path and sets ZCCACHE_DIR in $GITHUB_ENV (so downstream workflows can reference it in actions/cache's path: block).
2. The action README / fbuild docs include a canonical cache block:

- name: Setup fbuild
  id: fbuild-setup
  uses: fastled/fbuild/.github/actions/setup@main

- uses: actions/cache@v4
  with:
    path: |
      ~/.fbuild
      \${{ steps.fbuild-setup.outputs.zccache-store-path }}
      <project>/.build/pio/<board>
    key: fbuild-v1-\${{ runner.os }}-\${{ runner.arch }}-\${{ hashFiles('...') }}

Out of scope

• Whether caching zccache actually produces hits — that's build: audit zccache per-TU cache keys for cross-runner stability #150 (key stability audit). This issue is only about making the path visible so the next benchmark iteration can test it.
• zccache daemon pre-spawn warm-up — separate perf lever, tracked under [META] Fastest possible FastLED examples CI rebuild — profile + benchmark #112.

Definition of done

• setup action exposes zccache-store-path as a step output.
• A new README section in .github/actions/setup/README.md shows the canonical cache block.
• bench/fastled-examples iter-N picks up the output and caches the directory (tracked in bench: add zccache store to bench/fastled-examples actions/cache path list #151).