fix(ci): purge stale LPC framework tarball from restored fbuild cache

[zackees]

PRs #3543 and #3547 did not resolve the LPC-lane failures. Root cause: the fbuild setup action's cache uses restore-keys that fall back to 'fbuild-----' — a prefix that ignores cache-key-extra. So an lpc804 job whose exact key misses can still restore an unrelated board's cache (observed in run 28544189852: lpc804 restored uno_r4_wifi's cache). That cache holds a pre-transfer framework-arduino-lpc8xx tarball whose expected sha256 no longer matches what GitHub now serves for commit 50d76e0.

Fix: add a small step between the fbuild-cache restore and the pio/compile invocation that deletes ONLY /fbuild-cache/platforms/FastLED-framework-arduino-lpc8xx/. Other cached artifacts are untouched. Non-LPC boards never had this subtree so the step is a no-op for them. Deleted entries are lazily re-populated by the next fbuild call, which now downloads the FastLED-org tarball and records the correct sha256.

Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Resolved a build/cache issue that could cause certain compilations to fail after restoring older cached artifacts.
  • Improved build reliability by automatically clearing stale cached platform files before continuing the setup process.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 0422e72 and bdf4749.

📒 Files selected for processing (1)

• .github/workflows/build_template.yml

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📝 Walkthrough

Walkthrough

Adds a new step to the fbuild cache setup in the GitHub Actions build workflow that checks for and deletes a stale LPC8xx PlatformIO framework platform tarball directory under the restored fbuild cache, preventing sha256 mismatches during subsequent build steps.

Changes

Workflow Cache Purge

Layer / File(s)	Summary
Stale cache purge step .github/workflows/build_template.yml	Adds a bash step (under set -euo pipefail) that conditionally deletes the LPC8xx arduino platform tarball subtree from the restored fbuild cache directory.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• FastLED/FastLED#2319: Both PRs modify the fbuild cache handling logic in build_template.yml, with this PR further hardening the cache restore logic added in #2319.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/purge-stale-lpc-framework-tarball

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