Update Gemfile for Zuulv3

The logic in the Gemfile was relying on Zuulv2 variables to find out
whether the spec helper gem was already available on disk, and since
Zuulv3 has changed things it was failing to find it and downloading the
master version instead. This patch ensures the Gemfile looks for the gem
in the right place when running in CI.

Change-Id: I4a51af700fe42251306e527cac4170f3ea43fb75

Gemfile

@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
-if ENV['ZUUL_REF'] && File.exists?("#{ENV['WORKSPACE']}/openstack-infra/puppet-openstack_infra_spec_helper")
-  gem_checkout_method = {:path => "#{ENV['WORKSPACE']}/openstack-infra/puppet-openstack_infra_spec_helper"}
+if File.exists?('/home/zuul/src/git.openstack.org/openstack-infra/puppet-openstack_infra_spec_helper')
+  gem_checkout_method = {:path => '/home/zuul/src/git.openstack.org/openstack-infra/puppet-openstack_infra_spec_helper'}
 else
   gem_checkout_method = {:git => 'https://git.openstack.org/openstack-infra/puppet-openstack_infra_spec_helper'}
 end