v1.5.0: Add prefix input

Prefixing the Julia command

In some packages, you may want to prefix the julia command with another command, e.g. for running tests of certain graphical libraries with xvfb-run.
In that case, you can add an input called prefix containing the command that will be inserted to your workflow:

      - uses: julia-actions/julia-runtest@v1
        with:
          prefix: xvfb-run

If you only want to add this prefix on certain builds, you can include additional values into a combination of your build matrix, e.g.:

    strategy:
      fail-fast: false
      matrix:
        os: [ubuntu-latest, windows-latest, macOS-latest]
        version: ['1.0', '1', 'nightly']
        arch: [x64]
        include:
          - os: ubuntu-latest
            prefix: xvfb-run
    steps:
    # ...
      - uses: julia-actions/julia-runtest@v1
        with:
          prefix: ${{ matrix.prefix }}
    # ...

This will add the prefix xvfb-run to all builds where the os is ubuntu-latest.

---

See #24.

Thanks to everyone involved in adding and testing the feature :)