fix build esp32 with gn

[rwalker-apple]

Problem

pigweed and esp-idf fight over python library versions

Summary of Changes

limit pigweed's involvement in the build to where it has to be in the environment for 'gn gen'

with these changes, this works:

$ cd ~/dev/chip/examples/wifi-echo/server/esp32 && IDF_PATH=~/dev/esp-idf ./idf.sh make V=1 CHIP_BUILD_WITH_GN=y defconfig all

[rwalker-apple]

@bzbarsky-apple ?