Add GitHub Action for Flux CLI

[stefanprodan]

This PR adds a GitHub Action for using Flux CLI in CI.

Example workflow:

name: e2e
on:
  push:
    branches:
      - '*'
jobs:
  kubernetes:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v2
      - name: Setup Flux CLI
        uses: fluxcd/flux2/action@main
      - name: Setup Kubernetes Kind
        uses: engineerd/setup-kind@v0.5.0
      - name: Install Flux in Kubernetes Kind
        run: flux install