ci: Cache Electron downloads for PR tests (#336166)

Cache Electron downloads for PR tests

Warm platform-specific Electron archive caches on main and restore them in PR tests through a shared setup action. Preserve checksum validation, retries, and lookup-only dependency checks for already-warm jobs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/node_modules_cache/README.md

@@ -33,3 +33,39 @@ Example usage from a workflow step:
 If you change the archive format or flags, update both the `archive` and
 `extract` branch of the script, and bump `build/.cachesalt` to invalidate
 existing caches.
+
+## Electron download cache
+
+The platform jobs in [pr-node-modules.yml](../pr-node-modules.yml) also warm a
+separate Electron download cache on `main`. The
+[setup-electron](../../actions/setup-electron/action.yml) action shares cache
+handling and download retries with the Linux, macOS, and Windows PR test
+workflows:
+
+- By default, restore cached archives and validate/prepare Electron, downloading
+  with up to three attempts as needed. PR jobs use this mode without saving.
+- `lookup-only: 'true'` only checks for a cache entry, without restoring archives,
+  preparing Electron, or saving. This mode does not require npm dependencies.
+- `save-cache: 'true'` also saves after successful setup on a cache miss.
+
+The cache stores `@electron/get`'s original download archives, not the prepared
+`.build/electron` application. Its key includes the runner OS, target
+architecture, and a hash of `.npmrc` and `build/checksums/electron.txt`. Ordinary
+`package-lock.json` changes do not invalidate it. If a downloader update changes
+the cache format, bump the `electron-download-v1` prefix in `setup-electron`.
+
+The paths match `@electron/get`'s platform defaults, including `XDG_CACHE_HOME`
+on Linux and `LOCALAPPDATA` on Windows. Cache paths and keys are handled within
+the action.
+
+Even on a cache hit, PR jobs run `npm run electron` to validate and prepare the
+application from the cached archives using the current checkout. Checksum
+validation remains enabled: the current downloader still fetches
+`SHASUMS256.txt` from GitHub, so this cache avoids the large ZIP downloads but
+does not make Electron setup entirely network-independent.
+
+On `main`, an Electron cache hit uses lookup-only checks and skips downloading
+Electron. On a miss, the platform job restores `node_modules` (or installs them
+if that cache also misses), then invokes `setup-electron` with `save-cache: 'true'`.
+This checks the cache again before preparing and saving Electron, while keeping
+already-warm jobs from extracting dependency archives unnecessarily.