Support GOEXPERIMENT at build time

[nlnw]

It doesn't seem like GOEXPERIMENT works when you set it as an env var for an app. I tried setting GOEXPERIMENT=jsonv2 but the build fails because it doesn't detect that flag.

Can this please be fixed.

[nlnw]

Although maybe this is a bug in general. Why does the build process not pickup the env vars that are set on the app?