Disable RubyGems in gitlab-shell scripts for performance

By default, RubyGems is enabled, which causes the Ruby interpreter to load 600+ additional files at startup when gitlab-shell is designed not to use any external gems.

We tried to make this part of gitlab-shell source, but had to revert gitlab-shell!141 (merged) because it turns out Linux kernels don't allow shebangs with multiple arguments (e.g. #!/bin/env ruby --disable-gems). This is the simplest method to get it to work. Source installs will just be a bit slower unless this flag is set to the default via RUBYOPT or compiler flag.