Hash USER environment variable during system_uuid fallback

[nHackel]

I recently encountered a system where the USER environment variable got set to "domain/user". Such a name then gets interpreted as a filepath which breaks the system_uuid_fallback logic.

As a workaround this PR hashes the variable before using it in the filepath creation. Same user names should still result in the same hash values

[nHackel]

Ah actually the docs mention that the hash may change when a new process is started. I'm not entirely sure if that is an issue here, because the workers should be started before Dagger is loaded according to the docs. I also wasn't able to locally produce different hashes for the same value