Defer some preflight checks to @portage generation

Currently our preflight checks run whenever the user runs bazel, but
some checks are relevant only when the user generates @portage. This
patch moves them to @portage generation.

BUG=b:339769898
TEST=bazel build @alchemist//:alchemist
TEST=bazel build @portage//target/virtual/target-os  # fails
TEST=ALCHEMY_EXPERIMENTAL_OUTSIDE_CHROOT=1 bazel build \
     @portage//target/virtual/target-os  # passes

Change-Id: Id25ef752a6470af0443849112f6dc4c4007b2200
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/bazel/+/5526388
Commit-Queue: Raul Rangel <rrangel@chromium.org>
Tested-by: Shuhei Takahashi <nya@chromium.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
5 files changed