kivu-webcam/gstreamer_h264_webcam_encoding : fix test failing

gnome and chromium moved to core24 (on edge) and the test fails
with folowing error messages when the provider runs on 22.04:

07:59:58  gst-launch-1.0: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.38' not found
...

checkbox-provider-kivu/units/webcam/jobs.pxu

@@ -10,7 +10,11 @@ requires:
   package.name == "gstreamer1.0-vaapi"
 environ: GST_PLUGIN
 command:
-  source export_va_path.sh
+  # initially, we want to call export_va_path.sh to use the libva and VA drivers from chromium/gnome
+  # but this makes the tail fail when gnome moved to core24 and the provider is running on Jammy
+  #   gst-launch-1.0: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.38' not found
+  # so we skip export the paths and use the libva and VA drivers available on the host system
+  #source export_va_path.sh
   timeout 10 intel_gpu_top -J > "${PLAINBOX_SESSION_SHARE}"/gstreamer_raw_webcam_encoding_intel_gpu_top.json &
   gpu_top_pid=$!
   timeout 10 gst-launch-1.0 -v --gst-plugin-path="${GST_PLUGIN}" v4l2src ! videoconvert ! video/x-raw ! fakesink