pip_library: test for presence of --system option before invoking pip

[samwestmoreland]

Copy Chris's fix to pip_cmd from master to v1

The version of pip provided by python3-pip in Debian 12 will drop the custom --system option - passing it results in pip exiting with an error. Rather than always passing --system if the host OS is Debian-like, test whether the configured pip tool implements a --system option, and if so, pass it to the real pip invocation.

Fixes #81.