pkgs: add explicit 'meta.mainProgram'

Since upstream has decided to deprecate the implied value when using
`lib.getExe` (see [1]), we should set it explicitly.

[1]: https://github.com/NixOS/nixpkgs/pull/246386

pkgs/wifi-qr/default.nix

@@ -74,6 +74,7 @@ stdenvNoCC.mkDerivation rec {
     description = "WiFi password sharing via QR codes";
     homepage = "https://github.com/kokoye2007/wifi-qr";
     license = with licenses; [ gpl3Plus ];
+    mainProgram = "wifi-qr";
     maintainers = with maintainers; [ ambroisie ];
     platforms = platforms.linux;
   };