[Task]: Refactor Jenkins PostcommitJobBuilder to behave similarly as PrecommitJobBuilder

### What needs to happen?

Currently, PostcommitJobBuilder does not quite follow a Builder pattern (like PrecommitJobBuilder does) and it is inconvenient to add a new parameter (see discussion here: https://github.com/apache/beam/pull/26562#pullrequestreview-1416535133).

The task involves refactor the PostcommitJobBuilder.groovy in `.test-infra/jenkins` to a Builder pattern, and change the callers to use something like
```
PostcommitJobBuilder builder = new PostcommitJobBuilder( ...settings... )

builder.build {
closure
}
```
like what PrecommitJobBuilder does.

Note that we plan to migrate infrastructure to GitHub Action in the long term, so set this as a low priority task

### Issue Priority

Priority: 3 (nice-to-have improvement)

### Issue Components

- [ ] Component: Python SDK
- [ ] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner