Fix schema with detached steps

[xtexChooser]

commands is optional, when detach is set.

Also fixes #4065, which made commands required (it can be unset when entrypoint is available) and therefore some schema tests fail.

For a step like:

  detached:
    image: redis
    detach: true

It matches commands_step and plugin_step at the same time, so change oneOf to anyOf.

[anbraten]

Could you add some tests.

[xtexChooser]

There are already tests in test-step.yaml and test-dag.yaml, which are enough for these fixes.

[woodpecker-bot]

Tearing down https://woodpecker-ci-woodpecker-pr-4066.surge.sh