fix for failing workflows on github actions

[andrewgiuliani]

This PR fixes two issues with github actions that I've noticed:

The first problem is documented in issue #357. The unit tests now pass if I remove the with ScratchDir: from

simsopt/tests/mhd/test_spec.py

Line 141 in 38a20cb

with ScratchDir("."):

The second problem is documented in issue #405. For some reason, mayavi is having issues installing on some of the runners:
https://github.com/hiddenSymmetries/simsopt/actions/runs/8741345143/job/23987152753?pr=404
https://github.com/hiddenSymmetries/simsopt/actions/runs/8754269959/job/24025642995?pr=404

The error goes away by removing VIS from

simsopt/.github/workflows/tests.yml

Line 166 in 38a20cb

run: pip install -v ".[MPI,SPEC,VIS]"

since mayavi won't be installed.

Let's use this PR to discuss possible solutions, because the failing workflows are quite annoying. Removing VIS from above is not advisable, so we need to find an alternative way to fix the issue.