- Add modules argument description
- Describe how GO_PKGNAME works in modules mode; add note about GO_PKGNAME/GH_SUBDIR relationship
- Document GO_BUILDFLAGS
It does look like it is misnamed but technically it is a correct name. From go help install:
usage: go install [-i] [build flags] [packages] Install compiles and installs the packages named by the import paths.
GO_BUILDFLAGS is what gets passed as build flags argument.