Bump omnibus chef-infra to 15.4.20
Bump omnibus chef-infra to 15.4.20

chef-server builds began breaking when chef/omnibus-software#1099
introduced an appbundle of the inspec binary.

This pins chef-server's omnibus to a version of chef-infra works with
the omnibus-software change.

Signed-off-by: Christopher A. Snapp <dffaa860ed97a8fe2bd7fd9817f16f80af273fae@chef.io>

chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server