Specify waybar config in nix.

This is to facilitate having different waybar configs for each machine.

nix/configuration/configuration.nix

@@ -146,6 +146,22 @@
     #   })
     # ];
 
+    nixpkgs.overlays = [
+      (final: prev: {
+        imagemagick = prev.imagemagick.overrideAttrs (old: rec {
+          # 7.1.2-6 seems to no longer exist, so use 7.1.2-7
+          version = "7.1.2-7";
+
+          src = final.fetchFromGitHub {
+            owner = "ImageMagick";
+            repo = "ImageMagick";
+            tag = version;
+            hash = "sha256-9ARCYftoXiilpJoj+Y+aLCEqLmhHFYSrHfgA5DQHbGo=";
+          };
+        });
+      })
+    ];
+
     # This option defines the first version of NixOS you have installed on this particular machine,
     # and is used to maintain compatibility with application data (e.g. databases) created on older NixOS versions.
     #