crossbuild.bash: call "set -x" late

Fedora 28 executes A LOT of stuff from /usr/share/Modules/
on bash startup. Having -x in the shebang means we see
it all.

crossbuild.bash

@@ -1,9 +1,11 @@
-#!/bin/bash -eux
+#!/bin/bash -eu
 
 cd "$(dirname "$0")"
 
 B="go build -tags without_openssl"
 
+set -x
+
 GOOS=linux  GOARCH=arm   $B
 GOOS=linux  GOARCH=arm64 $B
 GOOS=darwin GOARCH=amd64 $B