We use semaphore CI with several parallel job and it looks like the environment
isn't detected properly currently:

- I couldn't find an environment variable giving the pull request number,
  (supposed to come from `PULL_REQUEST_NUMBER`), however I noted
  `SEMAPHORE_BRANCH_ID` which is the same ID between builds pushed to the same
   branch. Example value: 3385909.
- The `SEMAPHORE_BUILD_NUMBER` is the same across all workers for a given push,
  and changes at each push. It sounds like it should be used as `number`, not
  as `job`. This starts at 1 on each branch, and increases on each push.
- The job ID can be obtained via `SEMAPHORE_CURRENT_JOB`, and for a given build,
  each worker will have their own (e.g. 1, 2, 3, etc...). Looking at the config
  from the other CIs, I think it should be returned as `job`.

Improve earlier fix work with both Semaphore 2.0 and classic.