Create reusable workflows for scheduled and regular tests

[kratman]

Followup to #4172 since the focus of that ticket was making sure all the tests work.

Several of our CI workflows can be reworked into reusable workflows to avoid duplication. After #4172 it is pretty easy for unit/integration tests, but there are probably more that can be reworked. This will do a lot to simplify our CI process

[agriyakhetarpal]

The idea will be to call the reusable workflow in other workflows, i.e., to add a setup-pybamm workflow with workflow_call:, which takes in the following inputs:

1. caching: true/false (will control the caching for pybamm-requires and for pip in setup-python jobs),
2. idaklu: true/false (to not clone pybind11 if false),
3. and any other inputs that I may have missed

We can then call the workflow in a step in the scheduled tests and PR tests workflows, and then trigger nox as needed.

Longer term, this will be removed and we will switch to OS-specific workflows anyway in the coming months, so I have added a "low" priority label for now