overlays: add overlay for i3status-rust

overlays/default.nix

@@ -0,0 +1,3 @@
+{
+  i3status-rust = import ./i3status-rust;
+}

overlays/i3status-rust/default.nix

@@ -0,0 +1,19 @@
+final: prev:
+{
+  # overlay created because nixpkgs's `i3status-rust` depended on `notmuch`, and
+  # `notmuch`'s tests were briefly broken. the features I'm disabling, I don't
+  # need anyway: (at the time of writing)
+  #
+  # - notmuch
+  # - maildir
+  i3status-rust = prev.i3status-rust.overrideAttrs (oldAttrs: {
+    buildInputs = builtins.attrValues {
+      inherit (final)
+        dbus
+        openssl
+        pulseaudio
+      ;
+    };
+    cargoBuildFeatures = [ "pulseaudio" ];
+  });
+}