nixos/initrd-network-ssh: fix test

The test relied on moving `initrd` secrets from the store into the
`initrd` which was fine here as it's only an integration test and not a
production environment.

However, this broke in 20.09 when support for this was dropped[1]. To make
sure that the snakeoil key used as hostkey for `sshd` here actually gets
copied into the VM, I added a small script for this that takes care of
this process while building the initial ramdisk.

[1] d930466b77

nixos/tests/initrd-network-ssh/default.nix

@@ -22,6 +22,10 @@ import ../make-test-python.nix ({ lib, ... }:
             hostKeys = [ ./ssh_host_ed25519_key ];
           };
         };
+        boot.initrd.extraUtilsCommands = ''
+          mkdir -p $out/secrets/etc/ssh
+          cat "${./ssh_host_ed25519_key}" > $out/secrets/etc/ssh/sh_host_ed25519_key
+        '';
         boot.initrd.preLVMCommands = ''
           while true; do
             if [ -f fnord ]; then