tools (common/async_process): remove useless encoding + use full path for command: - arguments don't need to be encoded anymore, `spawnProcess` handles str directly now - use full path even for first argument: even if first argument should be only executable name, it appears that using full path only in `command` is not enough (seen with Python where system installation is used instead of the VirtualEnv one when full path is not used as first argument). (grafted from 25cbaf04772855cd8aead18771465a4f00f5fa03)