Cache Hermes build dir when building the test app

[kraenhansen]

In an attempt to speed up the native builds, I was contemplating caching the Hermes build directory.

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[kraenhansen]

For some reason the cache action fails to save the Hermes build directory on Android / Linux: https://github.com/callstackincubator/react-native-node-api/actions/runs/15834266404/job/44634111489?pr=138#step:33:3

Failed to save: Unable to reserve cache with key Linux-hermes, another job may be creating this cache.

I'll leave this PR in draft for a bit - but also don't want to sink too much time into this 😬