overlays: Patch fzf to prevent fzf_key_bindings error

I was going to wait until the next nixos-unstable release, but it's been
a week since the branch was updated. This commit makes it possible to
use the latest nixos-unstable release without worrying about the
fzf_key_bindings error introduced in the recent fzf update.

overlays/default.nix

@@ -1,6 +1,7 @@
 {
   imports = [
     ./alejandra.nix
+    ./fzf.nix
     ./zola.nix
   ];
 }

overlays/fzf.nix

@@ -0,0 +1,14 @@
+{
+  nixpkgs.overlays = [
+    (final: prev: {
+      fzf = prev.fzf.overrideAttrs (oldAttrs: {
+        postInstall = (oldAttrs.postInstall or "") + ''
+          cat << EOF > $out/share/fish/vendor_conf.d/load-fzf-key-bindings.fish
+            status is-interactive; or exit 0
+            fzf_key_bindings
+          EOF
+        '';
+      });
+    })
+  ];
+}