build_image: set keyset field in lsb-release

This tells dev built images to include CHROMEOS_RELEASE_KEYSET=devkeys.

BUG=chromium:443736
TEST=precq passes
CQ-DEPEND=CL:1217266, CL:*676258

Change-Id: Iadf178acccddeafb86e25954d8f4583caccaee15
Reviewed-on: https://chromium-review.googlesource.com/1217267
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
1 file changed