update_chroot: Don't record post-cross pkgs to world set

host-post-cross packages are toolchain packages and shouldn't be rebuilt
as part of regularly updating the @world set. There is a separate pass
in update_chroot to update toolchain packages when binary prebuilts are
available for new versions.

BUG=chromium:1012966
TEST=None

Change-Id: I896175b35df378f9b27c171edcdcc9cbc642606e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1853570
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/update_chroot b/update_chroot
index 6773daf..20b05d1 100755
--- a/update_chroot
+++ b/update_chroot
@@ -131,7 +131,7 @@
 # Install post cross packages if binary pkgs are available.
 if [ "${FLAGS_usepkg}" -eq "${FLAGS_TRUE}" ]; then
   # Use --usepkgonly to ensure that packages are not built from source.
-  EMERGE_FLAGS=( -uNv --with-bdeps=y --select --getbinpkg --deep --usepkgonly )
+  EMERGE_FLAGS=( -uNv --with-bdeps=y --oneshot --getbinpkg --deep --usepkgonly )
   EMERGE_FLAGS+=(
     $("${CHROMITE_BIN}/cros_setup_toolchains" --show-packages host-post-cross)
   )