ci: edit .drone.yml to only build for linux-amd64

* rm windows pipelines as I don't have any windows runners
* rm arm/arm64 pipelines as I don't have any arm runners
* rm {ecr,acr,whatever} publish steps as we're not publishing anything
  just yet
* tag the image under immawanderer, not the official plugins repo
* run as a dry_run (cause we're not really publishing, right?)

Author: wULLSnpAXbWZGYDYyhWTKKspEQoaYxXyhoisqHf