Github actions have undocumented environment variables named ACTIONS_RUNTIME_URL, ACTIONS_RUNTIME_TOKEN and ACTIONS_CACHE_URL. The actions/cache action relies on this to create the url to interact with for storing caches.
https://github.com/actions/toolkit/blob/1cc56db0ff126f4d65aeb83798852e02a2c180c3/packages/cache/src/internal/cacheHttpClient.ts#L33-L47

They all get populated over here

This is the cause of #285, #169 is linked.