implement GEM_HOME support

[Jon]

• never generate GEM_HOME on windows platforms under the assumption that gems will be installed into each ruby installation. This should be the 80%+ use case, but may need to revisit decision.
• do not generate a GEM_HOME for system ruby, but persist and restore any GEM_HOME that was active when uru admin add system install step was run.

[Jon]

Implement initial GEM_HOME behavior. Closes #7

The hard-coded windows behavior may become a source of teeth knashing and may need to be revisited after more usage.

Windows

Under the assumption that the vast majority of ruby on windows users install gems into the ruby installation, never generate a GEM_HOME env var and never add a GEM_HOME bindir to PATH. The single exception is during registration of a system ruby. If a user has set the GEM_HOME env var, its value will be persisted and used.

Unix-like

An unsurprising GEM_HOME value will be computed and persisted for each registered ruby of the form /home/.gem/$ruby/$version except for system rubies. Uru doesn't generate a GEM_HOME when registering the system ruby except if the user has already set the GEM_HOME env var. Uru takes a do-not-touch approach to system rubies and assumes the user has properly configured system (e.g. - added their GEM_HOME bindir to PATH).

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