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.