Touch soong_build output at the end of main

This fixes an incrementality bug that resulted in superfluous
build.ninja regeneration after a new glob definition is added.

(cherry picked from https://android-review.git.corp.google.com/c/platform/build/soong/+/2585669)
Fixes: 279674820
Test: Treehugger
Test; New integration test in this CL

Merged-In: Ifefe66a0eb1c125e9ad5373d60437a1cb1e6fdec
Change-Id: Ifefe66a0eb1c125e9ad5373d60437a1cb1e6fdec