Update GitHub action metadata to use ghcr.io image

The 'runs.image:' argument was using `Dockerfile`. This caused
a Docker build of the trufflehog image every time someone
used this action.

I've changed the argument to use the already published Docker image.
This will speed up the workflows considerably, since it pulls the
image that is already built and available on GitHub's image storage.

action.yml

@@ -17,7 +17,7 @@ branding:
   color: "green"
 runs:
   using: "docker"
-  image: "Dockerfile"
+  image: "docker://ghcr.io/trufflesecurity/trufflehog:latest"
   args:
     - git
     - file://${{ inputs.path }}