pw_env_setup: Switch from 'which' to 'command -v'
pw_env_setup: Switch from 'which' to 'command -v'

Change-Id: Ia24247a9a0580b88ccf57f134c8796951972797a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41345
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Paul Mathieu <8791b1cecbf562cc74a5b7acb099775bda2b56a0@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed