Allow gradual migration to go run

[qnighy]

It is now a standard practice to simply invoke go run, like go run github.com/golang/mock/mockgen, without using go build. At the same time, there is still a need to manage tool versions in go.mod, and gex may be useful without go build.

Especially, grapi depends on gex and needs a proper way out from the current go build-based workflow.

This PR introduces a new option --no-bin that stops gex from generating //go:generate go build comment. This flag is tracked by a newly generated comment line //gex:nobin in the manifest.

This way the user can gradually migrate from the go build-based workflow to the go run-based workflow.