tools/autopkgtest-build-lxd: pass /dev/null on stdin to lxc launch

Recent changes to lxd cause 'lxc launch' to fail if there is any input on
stdin queued, which is the case (at least) during the 'lxd' autopkgtest
included in this package, which calls autopkgtest-build-lxd from a here
document passed to a shell.  It may be the case for other external uses as
well, so it's best to leave the 'lxd' autopkgtest as is and fix the
autopkgtest-build-lxd script to use lxc launch with the new restriction.

The only other call to 'lxc launch' is from autopkgtest-virt-lxd, which
passes it to check_exec(), and that function explicitly sets stdin to
/dev/null, so no change is needed there.

(LP: #1845037)