specify workflow runs for testing on pull requests

[f-hafner]

Issue

Fixes #24

Description of changes

On draft pull requests, run a shorter version of the pipeline with only 1 OS and 1 python version. Based on https://github.com/dianna-ai/dianna/blob/main/.github/workflows/build.yml

Includes

• [X] Code changes
• [ ] Tests
• [ ] Documentation

[f-hafner]

tests are failing, but they also fail on master branch. let's wait until the tests pass on the master branch

[stefsmeets]

Some of the fixes in this PR were included in #62