[lldb/Test] Run dotest.py with the Python LLDB was built with.

The Python used to run lit can be different from the Python LLDB was
build with. One scenario where this happens is when LLVM can find the
Python 3 interpreter, but not the Python 3 libraries, in which case LLDB
build and links against Python 3. Without this change, you end up with
an ModuleNotFoundError because of the mismatch.

Instead of looking at the Python interpreter that's used to run lit,
lldbtest should use the interpreter that matches the Python version LLDB
was build against.

Differential revision: https://reviews.llvm.org/D79519