Add mypy-ribasim-qgis to github actions

[deltamarnix]

Fixes #1067

[deltamarnix]

@Hofer-Julian I fixed the mypy issue by changing the mypy_path. I put it in hardcoded now to see if it is working. If we want to maintain compatibility between 0.12.0 and 0.13.0 on different local machines:

• Put both the old and the new paths in the list, mypy won't complain.
• Do some magic mumbo jumbo with paths based on the version of pixi. There is the env_setup.bat which could look for pixi versions and change MYPYPATH env variable accordingly. It is only possible to use relative paths in MYPYPATH, so we would have to grab CONDA_PREFIX from the environment and strip off everything up to .pixi.

[Hofer-Julian]

Looks good!