CHROMIUM: Disable pretend phase when invoking ebuild

This changes disables the pretend phase from getting automatically
called when using the `ebuild package` command. We already disabled
this phase when calling emerge in CL:3388881.

BUG=b:280233260
TEST=rm -rf  /build/amd64-host/tmp/portage && 'ebuild' '--skip-manifest' '/mnt/host/source/src/third_party/eclass-overlay/acct-user/fwupd/fwupd-0.0.1.ebuild' 'package'

Change-Id: I96e5c326c9600aa87218fbf6b6973b4b01df7119
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/4877405
Tested-by: Raul Rangel <rrangel@chromium.org>
Commit-Queue: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Raul Rangel <rrangel@chromium.org>
1 file changed