Specifically, I'm using the magick package, and the features I'm using dictate that I always need to build it with the 'gm' build tag. Is there a way to declare that build tag by default, so that 'go build' for my project (or 'go test') will implicitly build magick using 'gm'?
No, they can't. You either have to type them on the cmd line or use a makefile.