simplify `--ci-mode` handling in incremental_buld.py

instead of copying from a run dir to logs later, do so
immediately after the cuj run (avoiding most of the magic
constants, except the "logs" directorry)

Bug:280492115
Test: locally ran `mkdir -p ~/timing/logs` then `USE_PERSISTENT_BAZEL=1 ./incremental_build.sh -b mixed_prod --ci-mode --cujs clean -l ~/timing nothing`
Change-Id: If8532c522131aee16acb2db7f6810bc2b00c47f4