ci-automation: Publish torcx_output_root to bincache

`build_image` depends on accesss to the torcx manifest and the "content
addressable nature" of the directory. We currently rely on the torcx output
root structure being preserved in the container image.

While we're moving the torcx output root out of the container image, preserve
its contents so that they can be restored from bincache.

ci-automation/image.sh

@@ -84,11 +84,17 @@ function _image_build_impl() {
             official_arg="--noofficial"
     fi
 
+    local torcx_root_tar="torcx_root.tar.zst"
     apply_local_patches
+    copy_from_buildcache "images/${arch}/${vernum}/torcx/${torcx_root_tar}" .
+
     # build image and related artifacts
     ./run_sdk_container -x ./ci-cleanup.sh -n "${image_container}" -C "${packages_image}" \
             -v "${vernum}" \
-            mkdir -p "${CONTAINER_IMAGE_ROOT}"
+            mkdir -p "${CONTAINER_IMAGE_ROOT}" "${CONTAINER_TORCX_ROOT}"
+    ./run_sdk_container -n "${image_container}" -C "${packages_image}" \
+            -v "${vernum}" \
+            tar --zstd -xf "${torcx_root_tar}" -C "${CONTAINER_TORCX_ROOT}"
     ./run_sdk_container -n "${image_container}" -C "${packages_image}" \
             -v "${vernum}" \
             ./set_official --board="${arch}-usr" "${official_arg}"

ci-automation/packages.sh

@@ -123,6 +123,11 @@ function _packages_build_impl() {
     # generate image + push to build cache
     docker_commit_to_buildcache "${packages_container}" "${packages_image}" "${docker_vernum}"
 
+    # publish torcx output root for consumption by build_image
+    local torcx_root_tar="torcx_root.tar.zst"
+    tar --zstd -cpf "${torcx_root_tar}" -C "${torcx_tmp}/torcx" .
+    copy_to_buildcache "images/${arch}/${vernum}/torcx" "${torcx_root_tar}"
+
     # Publish torcx manifest and docker tarball to "images" cache so tests can pull it later.
     create_digests "${SIGNER}" \
         "${torcx_tmp}/torcx/${arch}-usr/latest/torcx_manifest.json" \