make.bash: more robustly detect gold 2.20

On recent Debian systems the gold 2.20 check triggers though
Debian doesn't have version 2.20 but rather has:

        GNU gold (GNU Binutils for Debian 2.21.52.20110606) 1.11
                                                ^^^^

R=rsc, iant
CC=golang-dev
https://golang.org/cl/5252055

src/make.bash

@@ -10,7 +10,7 @@ if [ ! -f env.bash ]; then
 fi
 . ./env.bash
 
-if ld --version 2>&1 | grep 'gold.*2\.20' >/dev/null; then
+if ld --version 2>&1 | grep 'gold.* 2\.20' >/dev/null; then
 	echo 'ERROR: Your system has gold 2.20 installed.'
 	echo 'This version is shipped by Ubuntu even though'
 	echo 'it is known not to work on Ubuntu.'