CI: Run checks for each commit, not only the top one

[Davidson-Souza]

In a PR with multiple commits, we only run tests on the last one. But we should enforce that all commits build on themselves, this makes git bisection easier.

[luisschwab]

See https://github.com/bitcoin/bitcoin/blob/master/.github/workflows/ci.yml and https://github.com/bitcoin/bitcoin/blob/master/.github/ci-test-each-commit-exec.py.

Not sure if there's a more straight forward way to achieve this.