specify _class module attributes to explicitly declare module types

Should fix Fediversity/Fediversity#65 (comment)

Reviewed-on: Fediversity/Fediversity#403
Reviewed-by: kiara Grouwstra <kiara@procolix.eu>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-committed-by: Valentin Gagarin <valentin.gagarin@tweag.io>

default.nix

@@ -55,6 +55,7 @@ in
         };
       in
       [
+        pkgs.npins
         pkgs.nix-unit
         test-loop
       ];

panel/default.nix

@@ -27,7 +27,7 @@ in
       pkgs.openssh
     ];
     env = {
-      DEPLOYMENT_FLAKE = ../.;
+      DEPLOYMENT_FLAKE = toString ../.;
       DEPLOYMENT_NAME = "test";
       NPINS_DIRECTORY = toString ../npins;
       CREDENTIALS_DIRECTORY = toString ./.credentials;