Golang project's root may miss any .go files which makes build phase to
fail with error similar to:
no Go files in /tmp/<...>/src/golang.org/x/mod
This change implements a SKIP-BUILD? key parameter which is, by default,
set to #f to invoke build procedure. It is taken from cargo-build-system
* guix/build-system/go.scm (go-build, go-cross-build): Add "skip-build?"
key parameter.
* guix/build/go-build-system.scm (build): Add "skip-build?" key
parameter and implement logic.
Change-Id: I3f41414868a7329cbe99324106427cdae4884d94
c149fc769c