"python" command provided by pkg_alternatives is a shell script.
At least on NetBSD-7, execve can't execute scripts whose interpreter
is another shell script.  (While some "rich" shells like zsh seem
to have handle the case by itself, NetBSD's /bin/sh doesn't.)
Workaround the issue by using env command for shebangs for
these scripts.

Noticed with the recent tunnel-push-pop.at tests using ovs-pcap command.

Signed-off-by: YAMAMOTO Takashi <yamamoto@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>