Move build_hostname.txt generation to SetupOutDir

Before, build_hostname.txt was only written in runKati(). In soong-only
builds, runKati will not run, so build_hostname.txt wouldn't be
generated. Move it to SetupOutDir() which runs even in soong-only
builds.

Bug: 383902856
Test: m --soong-only dist
Change-Id: I7905581d47938d4060ff7a486ed9537c04dca820