pw_env_setup: Write gni file relative to root
pw_env_setup: Write gni file relative to root

Write environment.gni file relative to project root. When run locally
from the project root, this script places files in the expected
location. If run from outside the project root or in LUCI, the location
the gni file is placed was different because the current directory is
different.

Change-Id: I27d772407abfc1ccdbb489aeff549f09dea8e8fc
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/87263
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Reviewed-by: Erik Gilling <1100d3bbb092e6691a138903fe8ea324306d03ea@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