feat: expose cache-primary-key output (#597)

Expose the primary cache key computed by the caching logic as a new
`cache-primary-key` action output, so workflows can compose the built-in
setup-java cache key with actions/cache or actions/cache/restore across
steps and dependent jobs.

- src/cache.ts: set the `cache-primary-key` output in restore()
- action.yml: declare the new output
- README.md: document the new output
- __tests__/cache.test.ts: assert the output is set
- dist: rebuild

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

dist/setup/index.js

@@ -130957,6 +130957,7 @@ async function restore(id, cacheDependencyPath) {
     const primaryKey = await computeCacheKey(packageManager, cacheDependencyPath);
     core_debug(`primary key is ${primaryKey}`);
     saveState(STATE_CACHE_PRIMARY_KEY, primaryKey);
+    setOutput('cache-primary-key', primaryKey);
     // No "restoreKeys" is set, to start with a clear cache after dependency update (see https://github.com/actions/setup-java/issues/269)
     const matchedKey = await restoreCache(packageManager.path, primaryKey);
     if (matchedKey) {