Use tc.find() with the effective tool cache path

* So it finds already-download prebuilt Ruby in the GH-hosted tool cache path
  on self-hosted, where $RUNNER_TOOL_CACHE is often different than on GH-hosted.
* See #475.
* This relies on tc.find() looking up the env var every time:
  https://github.com/actions/toolkit/blob/1f4b3fac06/packages/tool-cache/src/tool-cache.ts#L522

windows.js

@@ -49,7 +49,7 @@ export async function install(platform, engine, version) {
 
   let rubyPrefix, inToolCache
   if (common.shouldUseToolCache(engine, version)) {
-    inToolCache = tc.find(common.engineToToolCacheName(engine), version)
+    inToolCache = common.toolCacheFind(engine, version)
     if (inToolCache) {
       rubyPrefix = inToolCache
     } else {