Ignore world set when updating portage for the SDK

Any missing packages in the world set will be reinstalled later in this
process, but @world blockers should be ignored when upgrading portage
itself at this stage.

TEST=`cros_sdk --replace`
TEST=chromiumos-sdk-tryjob
BUG=chromium:1026697

Change-Id: Ib8ec6a70a50479aef67ee8d36e652976e47fd97b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1926708
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed