[linux] suppress GEM_HOME env var when empty value

[Jon]

Creating an empty GEM_HOME= env var creates an empty GEM PATHS: slot visible when running gem env.

This env var is hard-coded in the bash runner script template, so have to figure out a diffferent way to inject it into the final runner script.

[Jon]

Suppress empty GEM_HOME. Partially fixes #14

→ <<cset 1e020eb1a749>>