Save compiled state into cache

When extracting from a binary download, the extracted file is cached:

```ts
installDir = await cacheDir(extPath, "grpc", versionSpec);
```

Can't we do the same after compiling ourselves?

It does look like something like this was intended, the compiled version is installed to `/opt/hostedtoolcache` (from `$AGENT_TOOLSDIRECTORY` env, it seems), but that isn't cached by default or automatically I think -- at least there is no github docs about that.

Even if I were to use `@actions/cache` to store and restore that folder, I'd still need to set the correct environment variables, so it would probably be easier if the script just did the caching logic itself and then set up the env.