buildGoModule: propagate setSourceRoot to goModules

The underlying goModules derivation must have the same source root as
the overall package. sourceRoot is being propagated already, but
setSourceRoot was not. This was surely an oversight.

This is implemented in such a way that it cannot break existing
vendorHashes unless the package is using setSourceRoot already. If
that's the case, then the package is either broken or found a workaround
to do deal with this very issue (e.g. by using `cd path` in an early
phase).

A simple search (`rg setSourceRoot $(rg -l buildGoModule)`) reveals that
no such packages obviously exist in Nixpkgs.