nix: separate into multiple overlays and expose them from top-level

nix/overlays/overrides.nix

@@ -0,0 +1,36 @@
+# This overlay is used for overriding upstream packages.
+
+self: super:
+
+let
+  nixpkgs-master = import <nixpkgs-master> { inherit (super) config; };
+
+in {
+  inherit nixpkgs-master;
+
+  # alacritty = nixpkgs-master.alacritty;
+  alacritty = super.stdenv.mkDerivation {
+    name = "alacritty-custom";
+    buildInputs = [ super.makeWrapper ];
+    phases = "installPhase";
+    installPhase = ''
+      makeWrapper ${self.nixpkgs-master.alacritty}/bin/alacritty $out/bin/alacritty \
+        --set-default WINIT_HIDPI_FACTOR 1.0
+    '';
+  };
+
+  roxterm = super.stdenv.mkDerivation {
+    name = "roxterm-custom";
+    buildInputs = [ super.makeWrapper ];
+    phases = "installPhase";
+    installPhase = ''
+      makeWrapper ${super.roxterm}/bin/roxterm $out/bin/roxterm \
+        --add-flags "--separate"
+    '';
+  };
+
+  # TODO: facetimehd is currfently broken (https://github.com/NixOS/nixpkgs/pull/72804)
+  facetimehd-firmware = super.hello;
+
+  qtile = self.nixpkgs-master.qtile;
+}