nixosTests.systemd-networkd: fix test flakiness

The original idea for this test was, on top of providing a networkd
test, to provide newcomers with a sample configuration they could use
to get started with networkd.

That's precisely why we were doing this systemd tmpfile dance in the
first place. It was a convenient way to create a runtime file with a
specific mode and owner.

Sadly, this tmpfile rule made the test flaky. There's a race condition
between the wireguard interface configured by systemd-networkd and
systemd-tmpfiles-setup.

Sometimes, networkd is going to try loading the wireguard private key
file *before* the said file gets created by systemd-tmpfiles.

A perfect solution here would be to create a "After" dependency
between wg0.netdev and systemd-tmpfiles-setup.service. Sadly, it is
currently impossible to create such a dependency between a
networkd-specific unit and a service.

We're removing this tmp file in favor of pointing networkd directly to
the Nix store. This is clearly something that shouldn't be done in the
real world for a private file: the store is world-readable. However,
this is the only way I found to fix this test flakiness for now.

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