Inject CGO_ENABLED env variable into build container

[mahendrapaipuri]

Proposal:
When we use promu with config cgo: true, inject an environment variable CGO_ENABLED=1 into the container.

Use case:
I have a monorepo that uses cgo to build couple of apps and pure go for others. I would like to build go and cgo apps from the same Makefile by looking at CGO_ENABLED env variable. golang-builder image exports this variable during build process but currently there is no way to inject env variables into the container during promu build/crossbuild commands.

@SuperQ Does it make sense injecting CGO_ENABLED=1 into container when cgo is set to true in promu config? If it does, I am happy to put up a PR. Cheers!