Only run job on specific branch with GitHub Actions

In a recent update you can now put if conditionals at job level. See the documentation here. https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idif

I tested this workflow which runs the job test on every push, but only runs deploy on the master branch.

name: my workflow
on: push
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - name: Execute tests
        run: exit 0
  deploy:
    runs-on: ubuntu-latest
    needs: test
    if: github.ref == 'refs/heads/master'
    steps:
      - name: Deploy app
        run: exit 0

What follows is my original answer, and an alternative solution if you prefer to have separate workflows.

The first workflow runs for every branch except master. In this workflow you run tests only.

on:
  push:
    branches:
      - '*'
      - '!master'

The second workflow runs for just master and runs both your tests and deploys if the tests were successfully passed.

on:
  push:
    branches:
      - master