fix(crypto/keyring): disallow non-owner reads of keyhash

[mark-rushakoff]

Description

The gosec linter has been complaining about this. On one hand, it's just a hash, but on the other hand, there is no apparent reason it needs to be readable by anyone other than the owner.

Also use the preexisting keyhashFilePath variable, instead of concatenating a forward-slash value, which may not work properly on Windows.

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[julienrbrt]

utACK.