Suggest branch restriction in GitHub Actions tutorial

[di]

I was working through the guide at https://packaging.python.org/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/ and made a mistake by creating a workflow that published any time a push is made.

The guide currently suggests:

on: push

But this means that the release job will run on any push on any branch.

I think instead, we should advise to restrict the job to the master branch:

on:
  push:
    branches:
      - master

[di]

(cc @webknjaz)

[webknjaz]

@di at the moment, I recommend making use of the environments — this would be more accurate as they allow having separate secrets and requiring approvals to run the corresponding jobs. I'd rather incorporate this in the tutorial.