Add workflow to test on latest pkgs versions

[Robinlovelace]

As suggested by @martinflies, it would be useful to test on dev versions of packages.

I think the following should do it:

name: Latest Versions
on:
  push:
    branches:
      - main

jobs:
  build:
    name: Build
    runs-on: ubuntu-latest
    container: ghcr.io/geocompx/docker:python
    defaults:
      run:
        shell: bash -l {0}
    env:
      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
    steps:
      - uses: actions/checkout@v2
      - name: Install Dependencies
        run: |
          conda env update --file environment.yml --prune
      - name: Render
        run: |
          # test code:
          quarto render
          ls docs