Keep Only editable workflow variables in bulk task process

In NXP-24659, we chose an approach to ignore the non editable workflow variables when using the bulk Process form: the goal was to be able to have a different behavior between the single Task process and the bulk task process.

However this approach does not cover the case of the workflow variables which does not take part in the resolution of the tasks. Nevertheless, because of the current code, these variables will be set on the node, and above all on the workflow. Therefore, when using the bulk process, the workflow variables from one document will overwrite the worflow variables of the others documents.

A different approach should be chosen: only keep the editable workflow variables from the bulk process form and use the variables from the task ifself

This way, we should be closer to the way wotkflow variables are handled in the single task process.