build: don't use a fixed filename in temp

Fixes #2688

R=golang-dev, gri, rsc
CC=golang-dev
https://golang.org/cl/5539050

doc/progs/run

@@ -68,7 +68,7 @@ for i in \
 done
 
 # Write to temporary file to avoid mingw bash bug.
-TMPFILE="/tmp/gotest3"
+TMPFILE="/tmp/gotest3.$USER"
 
 function testit {
 	$LD $1.$O