Create out dir for build_files mode

Previously, build_files required `timing` mode be run first (to create
the out dir). That was a bug.

Test: Manual run of build_files mode without timing mode first
Test: Run build_files with --skip-metrics and see the intended error
Change-Id: I4f978d1cacbb03e0408bc9e0e125e4f8c0ba90a6