Consistent trigger config for CI workflows

[bajtos]

Our repositories have a subtly different configuration of triggers for the CI workflow.

Let's agree on what the desired configuration is and update all CheckerNetwork repositories accordingly.

Links:

• ci: run for PRs targetting all branches piece-indexer#8
• ci: run CI for community-contributed pull requests spark-checker#128
• https://github.com/CheckerNetwork/spark-api/blob/main/.github/workflows/ci.yml
• https://github.com/CheckerNetwork/spark-evaluate/blob/main/.github/workflows/ci.yml
• https://github.com/CheckerNetwork/node/blob/main/.github/workflows/ci.yml