tmt/tests/build-image: Purge the image working root after the build

Once the image build is done, we do not need the working tree
anymore and leaving it around causes it to get archived at the
end of CI runs when it is totally unneeded.

tmt/tests/build-image.fmf

@@ -8,4 +8,5 @@ framework: shell
 path: /
 test: |
   ./kiwi-build --debug --image-type="$image_type" --image-profile="$image_profile" --kiwi-description-dir="${TMT_TREE}" --output-dir="${TMT_TEST_DATA}"
+  rm -rf "${TMT_TEST_DATA}/build/image-root"
 duration: 60m