environment: Use --use-existing-cipd

Use --use-existing-cipd in the environment module so we don't get flakes
when failing to download the cipd executable. This option was added by
pwrev/45021.

Change-Id: I423cce72fe9dd833e7c73c5ca2333680dc66a2dc
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/45160
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Oliver Newman <olivernewman@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>