Update runners away from discontinued `ubuntu-20.04`

[zachcran]

Description

We have CI actions pinned to the ubuntu-20.04 runner image, which was discontinued on April 15, 2025 by GitHub. This means that certain actions will not run anymore. These action should be updated to a more permanent runner image tag, like ubuntu-latest. This, combined with #166, will create more sustainable workflows while the regularly run tests will catch any issues caused by an image update.

Workflows to be updated

• .github/workflows/test_pip_install.yaml