cmd/go: variable built-in of "go generate"

[noliar]

I think $GOARCH and $GOOS in "go generate" should depend on the environment variable first, like other go commands.

[adg]

I can confirm:

$ cat gen.go 
package test

//go:generate echo $GOOS
$ go generate gen.go
darwin
$ GOOS=linux go generate gen.go
darwin

The code in cmd/go/generate.go uses runtime.GOARCH and runtime.GOOS for these values. It should probably use the values from the buildContext global variable.

[gopherbot]

CL https://golang.org/cl/11667 mentions this issue.