make.bash: show bootstrap version

For debugging. (The "go1.4" can be misleading since it might actually
be go1.4.3 or go1.11 or go1.12 or master)

Change-Id: I27520b931a2be018de577a299592d082260aa467
Reviewed-on: https://go-review.googlesource.com/c/go/+/204757
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/make.bash

@@ -162,7 +162,8 @@ IFS=$'\n'; for go_exe in $(type -ap go); do
 		fi
 	fi
 done; unset IFS
-echo "Building Go cmd/dist using $GOROOT_BOOTSTRAP."
+GOROOT_BOOTSTRAP_VERSION=$($GOROOT_BOOTSTRAP/bin/go version | sed 's/go version //')
+echo "Building Go cmd/dist using $GOROOT_BOOTSTRAP. ($GOROOT_BOOTSTRAP_VERSION)"
 if $verbose; then
 	echo cmd/dist
 fi