Use proper main location for building riff
Use proper main location for building riff

./main.go is a symlink to ./cmd/riff/main.go, it's intedned as a fall
back, but not the main entry point. We should use the proper entry point
when building riff.

Refs #1155

markfisher/sk8s,markfisher/sk8s,markfisher/sk8s,markfisher/sk8s