treewide: use 'lib.getExe' when possible

Don't use it in wireguard to keep it consistent, as only half the
commands could use it.

home/mail/accounts/default.nix

@@ -8,7 +8,7 @@ let
     realName = lib.mkDefault "Bruno BELANYI";
     userName = lib.mkDefault (mkMailAddress address domain);
     passwordCommand =
-      lib.mkDefault [ "${pkgs.ambroisie.bw-pass}/bin/bw-pass" "Mail" passName ];
+      lib.mkDefault [ (lib.getExe pkgs.ambroisie.bw-pass) "Mail" passName ];
 
     address = mkMailAddress address domain;
     aliases = builtins.map (lib.flip mkMailAddress domain) aliases;

home/mail/himalaya/default.nix

@@ -9,7 +9,7 @@ in
     settings = {
       notify-cmd =
         let
-          notify-send = "${pkgs.libnotify}/bin/notify-send";
+          notify-send = lib.getExe pkgs.libnotify;
         in
         pkgs.writeScript "mail-notifier" ''
           SENDER="$1"