Add missing sudo entry in gitian VM setup.

[ci skip]

doc/gitian-building.md

@@ -262,6 +262,7 @@ Then set up LXC and the rest with the following, which is a complex jumble of se
 # the version of lxc-start in Debian needs to run as root, so make sure
 # that the build script can execute it without providing a password
 echo "%sudo ALL=NOPASSWD: /usr/bin/lxc-start" > /etc/sudoers.d/gitian-lxc
+echo "%sudo ALL=NOPASSWD: /usr/bin/lxc-execute" >> /etc/sudoers.d/gitian-lxc
 # make /etc/rc.local script that sets up bridge between guest and host
 echo '#!/bin/sh -e' > /etc/rc.local
 echo 'brctl addbr br0' >> /etc/rc.local