build_library/build_image_util: disable ebuild-locks when merging binary packages

Disable ebuild-locks for the emerge command that creates the image.
Ebuild-locks protect unsandboxed ebuild phases from running
concurrently, but also slow things down greatly when a lot of
concurrency would otherwise be possible. The image build phase merges a
big amount of binary packages, and I am not aware of us having any
phases that risk concurrently modifying shared files.

I have been testing this for the last months and have not seen any
failures. The time savings are significant: this cuts image build time
from 20m to 10m for me.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

build_library/build_image_util.sh

@@ -155,6 +155,7 @@ emerge_to_image() {
   fi
 
   sudo -E ROOT="${root_fs_dir}" \
+      FEATURES="-ebuild-locks" \
       PORTAGE_CONFIGROOT="${BUILD_DIR}"/configroot \
       emerge --root-deps=rdeps --usepkgonly --jobs="${NUM_JOBS}" -v "$@"