[nearby] Update README to add build env script am: a3eb71b633

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2871060

Change-Id: I4bafffe88984d0e397f3c5a512e729745e20ee87
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>