release-testing: Use exec to create the log to not cover the exit code

examples/release-testing

@@ -2,6 +2,8 @@
 
 set -e
 
+exec 2>&1 | tee xen-tools-release-testing.log
+
 for dist in $(awk '!/^#|^$|dont-test/ {print $1}' /etc/xen-tools/distributions.conf); do
     xen-create-image --dist $dist --hostname "xen-tools-release-testing-$dist" --force --verbose "$@"
-done 2>&1 | tee xen-tools-release-testing.log
+done