Updating to PCB always resolves text variables from the schematic. This
means that back annotation will always start from resolved variables if
they exist in the schematic. So to avoid this, we prevent overwriting
any text variables that exist in the schematic.
This does prevent the user from updating a text variable in a pcbnew
field and backannotating this variable change into the schematic where
the schematic had a text variable already. Text variables are forward
annotation only until we add an additional property to each field to
include the unresolved text
Fixes https://gitlab.com/kicad/code/kicad/issues/21724
56ad08cdd8