Test using pip install Firedrake

[JHopeCollins]

Update our CI ready for pip install firedrake to land in firedrakeproject/firedrake#4011
See the equivalent Gusto PR for details firedrakeproject/gusto#618

[connorjward]

work out how to use a matrix strategy to reduce the boiler plate of running tests with each nprocs=? value

Unfortunately you can't use the matrix part of a workflow to make this happen because that produces multiple jobs (which start from scratch) whereas we need multiple steps. The best strategy might be another bash script.