crosvm: Add a note on cros_workon_make.
crosvm: Add a note on cros_workon_make.

It took me a while to realize the side effects making cargo fail.

BUG=None
TEST=read it.

Change-Id: I55db0e267452f865985d36d75c91a2314d117b06
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3026652
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm