Additional quote in shell executed process

https://github.com/boostorg/process/blob/6ccce9104ae91f6d07ac64c6d4e5fc0b57a649fe/include/boost/process/detail/posix/basic_cmd.hpp#L142
The quote around the last argument makes the shell interpret the entire command as a single command, unless it is quoted again, hence running `boost::process::system("echo any cmd", boost::process::shell)` will not work as expected.

I currently am using [this](https://github.com/w1d3m0d3/mpdfm/blob/1780495701d994a1add0122678f5399ce6c88519/src/config/config_file.cpp#L168-L178) as a workaround.