Rigify: fix the ORG bone rename affecting objects dependent on the rig.

The issue is that if a different object has a constraint or driver
targeting a control of the rig, renaming the bone after join when
re-generating the rig would change that target to the ORG bone.

To fix this, apply the renames to the temporary copy of the metarig
before joining it to the rig armature object. This also requires
moving the driver freeze to this stage.