nixos/dendrite: always substitute environment variables in config

Before this patch, services.dendrite.environmentFile is used for
secrets and environment variable substitution only happens when this
option is used.

systemd-247 provides a mechanism called LoadCredential for secrets and
it is better than environment file. See the section of Environment=
in the manual of systemd.exec for more information.

This patch always substitute environment variables, which enables the
usage of systemd LoadCredential.

nixos/modules/services/misc/dendrite.nix

···
         RuntimeDirectory = "dendrite";
         RuntimeDirectoryMode = "0700";
         EnvironmentFile = lib.mkIf (cfg.environmentFile != null) cfg.environmentFile;
-        ExecStartPre =
-          if (cfg.environmentFile != null) then ''
-            ${pkgs.envsubst}/bin/envsubst \
-              -i ${configurationYaml} \
-              -o /run/dendrite/dendrite.yaml
-          '' else ''
-            ${pkgs.coreutils}/bin/cp ${configurationYaml} /run/dendrite/dendrite.yaml
-          '';
+        ExecStartPre = ''
+          ${pkgs.envsubst}/bin/envsubst \
+            -i ${configurationYaml} \
+            -o /run/dendrite/dendrite.yaml
+        '';
         ExecStart = lib.strings.concatStringsSep " " ([
           "${pkgs.dendrite}/bin/dendrite-monolith-server"
           "--config /run/dendrite/dendrite.yaml"