Auto merge of #123451 - Kobzol:arbitrary-try-build, r=pietroalbini

CI: add a script for dynamically computing CI job matrix

It would be great if was easier to run specific CI workflows locally, and also to allow us to spawn a specific CI workflow by bors, to enable running arbitrary try builds. See discussion [here](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/CI.20workflows.20refactoring).

This PR is a first step in that direction.
- Moves the definition of CI runners and (for now) PR jobs into a separate `jobs.yml` file.
- Adds a simple Python script that reads the file, decides which jobs should be active for the current CI workflow, and prints them as JSON to their output.
- The PR job then reads this output and generates its job matrix based on it.

By moving the job definitions from `ci.yml` into a separate file, we can handle it programmatically, which should make it easier to both do local execution of CI jobs and also to do arbitrary try builds.

Author: bors