Fix path appending in copy_extra_files

Bug: 474214382
Test: On arm64 host: KOKORO_HOST_ROOT_DIR=$PWD KOKORO_ROOT_DIR=/tmp/build docker run -eKOKORO_HOST_ROOT_DIR -eKOKORO_ROOT_DIR -v/var/run/docker.sock:/var/run/docker.sock -v$PWD:/tmp/build --entrypoint /tmp/build/external/python/cpython3/kokoro/kokoro_build.sh docker:cli
Change-Id: I1941ab764a65d8f6d7f982f6e2d105d0d7f432c5