configure flakybot

I'd like to use flakybot so that we are alerted when we have a test failure https://github.com/googleapis/repo-automation-bots/tree/main/packages/flakybot

See these lines from https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/.kokoro/build.sh

```
# If this is a continuous build, send the test log to the FlakyBot.
# See https://github.com/googleapis/repo-automation-bots/tree/main/packages/flakybot.
if [[ $KOKORO_BUILD_ARTIFACTS_SUBDIR = *"continuous"* ]]; then
cleanup() {
chmod +x $KOKORO_GFILE_DIR/linux_amd64/flakybot
$KOKORO_GFILE_DIR/linux_amd64/flakybot
}
trap cleanup EXIT HUP
fi
```

I'd rather not migrate to "trampoline" as we have our own Docker based test cluster and our own service account. Instead, perhaps we can make the flakybot binary available some other way and allow our cluster service account to push to the bucket?