pw_env_setup: Allow using existing cipd client

Add --use-existing-cipd which will cause the cipd part of env setup to
use a 'cipd' executable already in PATH instead of downloading it anew.
This will be used in CI/CQ where pulling down the cipd client is an
infrequent but occasional failure, and there's a guarantee of an
existing 'cipd' executable anyway.

Tested by adding --use-existing-cipd to bootstrap.sh and it worked. Then
removed because most Pigweed developers won't have 'cipd' already
somewhere else in their PATH.

Change-Id: Ieb5d505c47211e1c63338067ccd7de2006a0ad82
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/45021
Commit-Queue: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>