cmd/dist: include "cmd/buildid" in toolchain in debug mode

Fixes #22599.

Change-Id: I2d59a8fae457881f681184fc6ed1f2aa597699b3
Reviewed-on: https://go-review.googlesource.com/76026
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/build.go

@@ -1118,6 +1118,11 @@ func cmdbootstrap() {
 
 	xflagparse(0)
 
+	if debug {
+		// cmd/buildid is used in debug mode.
+		toolchain = append(toolchain, "cmd/buildid")
+	}
+
 	if isdir(pathf("%s/src/pkg", goroot)) {
 		fatalf("\n\n"+
 			"The Go package sources have moved to $GOROOT/src.\n"+