Correct Mac offline installer signing

Keychain can be unlocked only once, and there are several installers to be signed.

Change-Id: Id851ad9c67cfc10b42e08cfeb766cb742fd8ae90
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>