wireguard: T2226: fix generation of psk
wireguard: T2226: fix generation of psk

Commit 264e0059449 ("util: T2226: rewrite wireguard oper to use run") changed
the code on PSK key generation, but the generated key must be output to
stdout.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x