refactor: extract git config

flake.nix

@@ -28,6 +28,7 @@
             user = "mandlm";
             userName = "Michael Mandl";
             userEmail = "mandlm@molez.net";
+            userSigningKey = "4AA25D647AA54CC7";
             theme = "dark";
           };
         };
@@ -42,6 +43,7 @@
             user = "mmandl";
             userName = "Michael Mandl";
             userEmail = "michael.mandl@horsch.com";
+            userSigningKey = "088ED38F036C7AF2";
             theme = "dark";
           };
         };