Effectively disable network access during the build

This starts a new network namespace without any connections to the
outside.

Bug: 122270019
Test: USE_GOMA=true m libc
Test: treehugger
Test: add rule to use /usr/bin/wget, fails after this change
Change-Id: Iba262025ce0e4e3bef5c34c817cc678d6c61403b