pw_env_setup: Add GN vars example to docs
pw_env_setup: Add GN vars example to docs

Change-Id: Idc50fb1ac40a7ea61f9e2d5610dc338e4ef8cbb1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/88720
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Reviewed-by: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

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