Merge per job env variables with job variables from matrix strategy

You may want to be able to specify some env variables for all jobs, but some configurations needs additional env variables

on: push
jobs:
  _:
    strategy:
      matrix:
        os:
        - ubuntu-latest
        - macos-latest
        include:
        - os: ubuntu-latest # Only ubuntu-latest has an extra env var MYVAR
          env:
            MYVAR: myval
    env:
      SOME_ENV: B
      ${{ insert }}: ${{ matrix.env || fromjson('{}') }}
    runs-on: ${{ matrix.os }}
    steps:
    - run: env