[proxy] have symlinking respond to WEAVE_DEBUG not DEBUG

prog/weaveexec/symlink

@@ -4,7 +4,7 @@
 
 set -e
 
-[ -z "$DEBUG" ] || set -x
+[ -z "$WEAVE_DEBUG" ] || set -x
 
 target="$1"
 user="$(stat -c '%u:%g' "$target")"

proxy/proxy.go

@@ -575,8 +575,8 @@ func (proxy *Proxy) symlink(unixAddrs []string) (err error) {
 		"PATH=/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
 	}
 
-	if val := os.Getenv("DEBUG"); val != "" {
-		env = append(env, fmt.Sprintf("%s=%s", "DEBUG", val))
+	if val := os.Getenv("WEAVE_DEBUG"); val != "" {
+		env = append(env, fmt.Sprintf("%s=%s", "WEAVE_DEBUG", val))
 	}
 
 	container, err = proxy.client.CreateContainer(docker.CreateContainerOptions{