gpg-agent: add GNUPG_HOMEDIR to environment (#1932)

otherwise, if `programs.gpg.homedir` is not set to default, calls to
`gpg` will fail to pick up anything related to secrets

modules/services/gpg-agent.nix

@@ -240,6 +240,7 @@ in
           ExecStart = "${pkgs.gnupg}/bin/gpg-agent --supervised"
             + optionalString cfg.verbose " --verbose";
           ExecReload = "${pkgs.gnupg}/bin/gpgconf --reload gpg-agent";
+          Environment = "GNUPGHOME=${homedir}";
         };
       };