prod_image_util: unpack SLSA report along with gcc when building image

Prod images need libstdc++.so and other libraries produced by
sys-devel/gcc build, but because we don't want all of gcc in the image,
the binpkg is manually unpacked instead of installed with emerge. Make
sure to preserve SLSA metadata when unpacking as well.

build_library/prod_image_util.sh

@@ -46,7 +46,8 @@ extract_prod_gcc() {
     qtbz2 -O -t "${pkg}" | \
         sudo tar -C "${root_fs_dir}" -xj \
         --transform 's#/usr/lib/.*/#/usr/lib64/#' \
-        --wildcards './usr/lib/gcc/*.so*'
+        --wildcards './usr/lib/gcc/*.so*' \
+        --wildcards './usr/share/SLSA'
 
     package_provided "${gcc}"
 }