Similar to the requested support for environment variables in if conditions, It would be nice to be able to use environment variables in jobs.<job_id>.steps.with 

This currently fails, see for example ergebnis/php-package-template#203:

on:
  - pull_request
  - push
name: "Continuous Integration"
env:

REQUIRED_PHP_EXTENSIONS: "mbstring"
jobs:

coding-standards:

name: "Coding Standards"
runs-on: ubuntu-latest

steps:
  - name: "Checkout"
    uses: actions/checkout@v2.0.0

  - name: "Install PHP with extensions"
    uses: shivammathur/setup-php@1.6.1
    with:
      coverage: none
      extensions: "$REQUIRED_PHP_EXTENSIONS"
      php-version: 7.2