update_chroot: Fix /etc/portage/patches symlink

This was accidentally broken by #3795. I took that change from a branch
where the `COREOS_OVERLAY` variable was no longer used at all.

Signed-off-by: James Le Cuirot <jlecuirot@microsoft.com>

Author: chewi

update_chroot

@@ -80,7 +80,7 @@ info "Setting up portage..."
 sudo mkdir -p "${REPO_CACHE_DIR}/distfiles"
 sudo chown "${PORTAGE_USERNAME}:portage" "${REPO_CACHE_DIR}/distfiles"
 sudo mkdir -p /etc/portage/repos.conf /var/lib/portage/pkgs
-sudo ln -sfT "${COREOS_OVERLAY}/coreos/user-patches" '/etc/portage/patches'
+sudo ln -sfT "${REPO_ROOT}/src/third_party/coreos-overlay/coreos/user-patches" /etc/portage/patches
 sudo touch /etc/portage/make.conf.user
 
 sudo_clobber "/etc/portage/make.conf" <<EOF