(maint) use $ruby_arch only if it has been set

In ci-bootstrap-from-artifacts.sh, we ran into an issue where the script
will attempt to append $ruby_arch if the platform is Windows, even if
Jenkins has not set $ruby_arch. This adds a sanity check to ensure
$ruby_arch is set and has a value before trying to use it.
Merge pull request #2922 from rji/qeng-1013/master/modify-acceptance-script

(maint) use $ruby_arch only if it has been set