cmd/dist: move GOPATH setting from init to bootstrap

CL 168697 unconditionally set GOPATH in dist, which broke the
misc-vet-vetall builder, because cmd/vet/all depends on GOPATH.

Fixes #30971

Change-Id: If6a58e054c6a4fedc2ea506a2c443348489c91f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/169217
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/dist/build.go

@@ -235,13 +235,6 @@ func xinit() {
 	// make.bash really does start from a clean slate.
 	os.Setenv("GOCACHE", pathf("%s/pkg/obj/go-build", goroot))
 
-	// Set GOPATH to an internal directory. We shouldn't actually
-	// need to store files here, since the toolchain won't
-	// depend on modules outside of vendor directories, but if
-	// GOPATH points somewhere else (e.g., to GOROOT), the
-	// go tool may complain.
-	os.Setenv("GOPATH", pathf("%s/pkg/obj/gopath", goroot))
-
 	// Make the environment more predictable.
 	os.Setenv("LANG", "C")
 	os.Setenv("LANGUAGE", "en_US.UTF8")
@@ -1211,6 +1204,13 @@ func cmdbootstrap() {
 
 	xflagparse(0)
 
+	// Set GOPATH to an internal directory. We shouldn't actually
+	// need to store files here, since the toolchain won't
+	// depend on modules outside of vendor directories, but if
+	// GOPATH points somewhere else (e.g., to GOROOT), the
+	// go tool may complain.
+	os.Setenv("GOPATH", pathf("%s/pkg/obj/gopath", goroot))
+
 	if debug {
 		// cmd/buildid is used in debug mode.
 		toolchain = append(toolchain, "cmd/buildid")