CI workflow is very slow

[emileten]

Is your feature request related to a problem? Please describe.

The CI workflow, which only installs dependencies and runs formatting, linting and short tests, takes almost 15 minutes. Most of that time seems to be taken by conda installations.

Describe the solution you'd like

Speed up CI. It should be possible, since this type of flow with different tools is pretty fast locally (~2 minutes)

• pip install all dependencies (including dev) locally in a virtual environment]
• run linting, formatting and test scripts