Building package before codecgen generation breaks if types changed

[2opremio]

#1042 builds a package before invoking codecgen to workaround ugorji/go#145

However, as @foot experienced, that's not enough: building the package will fail if the types changed since the generated codec will be outdated.

env -u GOARCH -u GOOS env GO15VENDOREXPERIMENT=1 GOGC=off go build -i -tags 'netgo unsafe' ./render/detailed
# github.com/weaveworks/scope/render
render/render.codecgen.go:56: undefined: report.NodeSet
render/render.codecgen.go:1525: undefined: report.NodeSet
render/render.codecgen.go:1765: undefined: report.NodeSet
Makefile:82: recipe for target 'render/detailed/detailed.codecgen.go' failed
make: *** [render/detailed/detailed.codecgen.go] Error 2
make: Leaving directory '/go/src/github.com/weaveworks/scope'
make: *** [render/detailed/detailed.codecgen.go] Error 2

Full logs: https://gist.github.com/foot/b0e70c7563b0fd2b80a5