*: set isSystemUser or isNormalUser

This is a requirement for nixpkgs 21.05, or else evaluation will fail
with:

Exactly one of users.users.${name}.isSystemUser and users.users.${name}.isNormalUser must be set.

krebs/3modules/exim.nix

@@ -78,6 +78,7 @@ in {
         inherit (cfg.user) home name uid;
         createHome = true;
         group = cfg.group.name;
+        isSystemUser = true;
       };
     };
   };

krebs/3modules/git.nix

@@ -366,6 +366,7 @@ let
         # To allow running cgit-clear-cache via hooks.
         cfg.cgit.fcgiwrap.group.name
       ];
+      isSystemUser = true;
       shell = "/bin/sh";
       openssh.authorizedKeys.keys =
         unique
@@ -384,6 +385,7 @@ let
       users.${cfg.cgit.fcgiwrap.user.name} = {
         inherit (cfg.cgit.fcgiwrap.user) home name uid;
         group = cfg.cgit.fcgiwrap.group.name;
+        isSystemUser = true;
       };
     };
 

krebs/3modules/htgen.nix

@@ -66,6 +66,7 @@ let
       nameValuePair htgen.user.name {
         inherit (htgen.user) home name uid;
         createHome = true;
+        isSystemUser = true;
       }
     ) cfg;
 

krebs/3modules/tinc.nix

@@ -236,6 +236,7 @@ let
       nameValuePair "${netname}" {
         inherit (cfg.user) home name uid;
         createHome = true;
+        isSystemUser = true;
       }
     ) config.krebs.tinc;
 

tv/2configs/pulse.nix

@@ -116,6 +116,7 @@ in
       group = "pulse";
       extraGroups = [ "audio" ];
       home = "${runDir}/home";
+      isSystemUser = true;
     };
   };
 }