treewide: ensure services are started after secrets setup

Now all services that access secrets only run after the secrets setup
has finished.

Previously, we assumed that the systemd `after` dependency is
transitive, i.e. that adding an `after = [ "bitcoind.service" ]`
to a service implicitly pulled in the `after` dependency to
`nix-bitcoin-secrets.target` (which is defined for `bitcoind`).
This is not the case. Services could start before secrets setup
had finished, leading to service failure.

modules/btcpayserver.nix

@@ -174,7 +174,7 @@ in {
     in rec {
       wantedBy = [ "multi-user.target" ];
       requires = [ "bitcoind.service" "postgresql.service" ] ++ optional cfg.btcpayserver.lbtc "liquidd.service";
-      after = requires;
+      after = requires ++ [ "nix-bitcoin-secrets.target" ];
       preStart = ''
         install -m 600 ${configFile} '${cfg.nbxplorer.dataDir}/settings.config'
         {