zoom-us: unset Qt env variables to fix dialog boxes

Zoom wouldn't show the "Participants"
dialog box when used in a plasma environment;
precisely, the dialog box failed to show its content.
The problem doesn't exist in other
environments like Gnome or Xfce.
Experiments have shown that clearing the environment variable
`QML2_IMPORT_PATH` before calling Zoom fixes the issue.

I suspect the reason to be as follows:
While the zoom build recipe is called with
`libsForQt5xx.callPackage`, putting `qttools.dev` in zoom's
`PATH` is the only connection to nixpkgs' Qt ecosystem.
Zoom brings its own Qt libraries.
Hence it seems to be a good idea to shield
zoom from access to nixpkgs' Qt files to avoid
problems from version mismatch or similar troubles.
So the commit at hand expands zoom's wrapper script
to clear the Qt-related enviornemt variables
`QML2_IMPORT_PATH` and `QT_PLUGIN_PATH`.

Original issue report, with some discussion:
https://github.com/NixOS/nixpkgs/issues/107495#issuecomment-764538071

pkgs/applications/networking/instant-messengers/zoom-us/default.nix

@@ -101,8 +101,12 @@ in stdenv.mkDerivation rec {
     rm $out/bin/zoom
     # Zoom expects "zopen" executable (needed for web login) to be present in CWD. Or does it expect
     # everybody runs Zoom only after cd to Zoom package directory? Anyway, :facepalm:
+    # Also clear Qt environment variables to prevent
+    # zoom from tripping over "foreign" Qt ressources.
     makeWrapper $out/opt/zoom/ZoomLauncher $out/bin/zoom \
       --run "cd $out/opt/zoom" \
+      --unset QML2_IMPORT_PATH \
+      --unset QT_PLUGIN_PATH \
       --prefix PATH : ${lib.makeBinPath [ coreutils glib.dev pciutils procps qttools.dev util-linux ]} \
       --prefix LD_LIBRARY_PATH ":" ${libs}