Always use out/host for host tools

Previously, files were installed to out/host or out/soong/host based
on if kati was used or not. That was changed in aosp/3418304, but
this HostToolDir() that's used by blueprint was not updated to match
that change, so soong-only builds were failing with missing host tools.

Bug: 388038396
Test: m --soong-only dist, ./prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ib104a6399ce2308d81651ec620323caaf9c76151