CI: make actions locally testable

[Calychas]

One can use https://github.com/nektos/act to test workflows locally, but the may need some tweaking to make them work.

See also https://www.redhat.com/en/blog/testing-github-actions-locally for additional improvements (e.g. mocking gh api)

Motivation:
Currently to test gh releases I needed to create a fork https://github.com/kraina-ai/srai-sandbox, which is a bit obrzydliwe (eng. disgusting). Other actions need commits to remote repo and can take quite some time to get a response.

[Calychas]

See also how the playground approach is done in https://github.com/process-analytics/GitHub-Actions-playground