automatically start/upgrade buildkitd

- Automatically start a buildkit daemon if no BUILDKIT_HOST is
  provided (and if not already started)
- Customization of BUILDKIT_HOST is still possible, just like before
- Automatically upgrade the managed daemon to the version used by dagger
  if necessary
- Add CI test to make sure the managed buildkit and the vendored
  buildkit versions match

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

.github/workflows/ci.yml

@@ -35,10 +35,6 @@ jobs:
         run: |
           make lint
 
-      - name: Start buildkit
-        run: |
-          docker run -d --name buildkitd --privileged moby/buildkit:v0.8.2
-
       - name: Integration test
         run: |
           make integration