Enable RPM signing in OLD CI’s build script

Omnibus 4.0+ determines if a `*.rpm` should be signed by if a signing passphrase has been set. We pass this passphrase into our Omnibus projects via the `OMNIBUS_RPM_SIGNING_PASSPHRASE` environment. If this environment variable is not set signing is effectively disabled as `ENV['OMNIBUS_RPM_SIGNING_PASSPHRASE']` returns nil.

/cc @opscode/release-engineers @opscode/client-engineers @jtimberman