Specify runs-on at the workflow level

Just a minor issue, but in a workflow where every job uses the same virtual environment;  it would be simpler to specify runs-on once at the top-level, rather than having to repeat the same value in every job.

e.g. instead of this:

name: CI
...
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    ...

  test:
    runs-on: ubuntu-latest
    steps:
    ...

  publish:
    runs-on: ubuntu-latest
    step:
    ...

…you could simply do this:

name: CI
runs-on: ubuntu-latest
...
jobs:
  build:
    steps:
    ...

  test:
    steps:
    ...

  publish:
    step:
    ...

Any job that doesn’t specify it’s own runs-on value would inherit the runs-on value for the workflow.

Not only “runs-on” but also envrionment variables.

Environment variables are now supported at both the workflow and job level https://help.github.com/en/github/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#env.

I have captured the feedback about runs-on and will consider it for the future.