[bug] No keep-alives sent when private is set via PostUp

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello,

I've encountered an edge case where no keep alives would be sent
following recreating a connection with

--8<---------------cut here---------------start------------->8---
wg-quick down my-config-file
wg-quick up my-config-file
--8<---------------cut here---------------end--------------->8---

Where my-config-file contains something like:

--8<---------------cut here---------------start------------->8---
cat /gnu/store/zilv4f0jqa8nz8apqv8y3a6g0ifymxhc-wireguard-config/wg0.conf 
[Interface]
Address = 10.0.0.7/32
Table = auto

PostUp = /gnu/store/4cnl0h79zc599xryr5jh66d7yq643zk4-wireguard-tools-1.0.20210914/bin/wg set %i private-key /etc/wireguard/private.key

ListenPort = 51820

[Peer] #apteryx
PublicKey = JPWIbC9qMlnTkWfqGp0plOxWJ/ewOO/C9BuxIJles28=
AllowedIPs = 10.0.1.1/32
Endpoint = apteryx.duckdns.org:51820
PersistentKeepalive = 25
--8<---------------cut here---------------end--------------->8---

The following command on that machine:
--8<---------------cut here---------------start------------->8---
tcpdump -n -i any port 51820
--8<---------------cut here---------------end--------------->8---

wouldn't show any traffic.

Discussing this on #wireguard (libera.chat IRC), the another| user
thinks the problem could be triggered because of setting the private key
in using a PostUp directive; more specifically it is believed the
problem would happen when "no private key is defined when the interface
comes up".

-- 
Thanks,
Maxim