How to access matrix variables in Github actions

I've set up a Github workflow for my CI needs, and it seems to somehow reset values assigned to matrix variables to an empty string whenever I do a comparison using matrix values e.g

if: matrix.python-version == '3.8' && matrix.toxenv=='quality'

and I've specified runs-on: ${{ matrix.os }} it fails saying:

Error when evaluating 'runs-on' for job 'run_tests'. (Line: 12, Col: 14): Unexpected value ''

if I specify runs-on: ubuntu-20.04 it works just fine.

complete Github workflow: (I've setup TOXENV environment variables so it runs a new job for each tox environment in parallel, and to avoid unnecessary builds running with every job (e.g quality) and slowing down the jobs)

on:
  push:
    branches: [ master ]
  pull_request:
    branches: [ master ]

jobs:
  run_tests:
    name: Tests
    runs-on: ${{ matrix.os }}
    strategy:
      matrix:
        os: [ubuntu-20.04]
        python-version: ['3.5', '3.8', '3.9']
        toxenv: ['django22', 'django30','django31']
        include:
          - python-version: "3.8"
            toxenv: "quality"
        exclude:
          - python-version: "3.5"
            toxenv: "django30"
          - python-version: "3.5"
            toxenv: "django31"

    steps:
    - uses: actions/checkout@v2
    - name: setup python
      uses: actions/setup-python@v2
      with:
        python-version: ${{ matrix.python-version }}

    - name: Install pip
      run: pip install -r requirements/pip.txt

    - name: Install Dependencies
      run: pip install -r requirements/ci.txt

    - name: Run Tests
      env:
        TOXENV: python${{ matrix.python-version }}-${{ matrix.toxenv }}
      run: tox

    # it'll run a separate job for quality checks with python 3.8
    - name: Run Quality
      if: matrix.python-version == '3.8' && matrix.toxenv=='quality'
      env:
        TOXENV: ${{ matrix.toxenv }}
      run: tox

    - name: Run Coverage
      if: matrix.python-version == '3.8' && matrix.toxenv=='django22'
      uses: codecov/codecov-action@v1
      with:
        flags: unittests
        fail_ci_if_error: true