[libcxx] [test] Quote the python executable in the executor (#68208)

This reapplies the change from
c218c80c, which was lost in the
refactoring in 78d649a4.

On Windows, the Python interpreter is by default installed in a path
like "C:\Program Files\Python38\python.exe", which requires quoting when
included in a concatenated command string (as opposed to a command list,
where each argument is a separate element).

This doesn't show up in the CI environments as they use Python installed
in a different directory, like C:\Python.

In addition to the python executable, also quote the path to run.py, if
needed.