[lldb/Utils] Use PYTHON_EXECUTABLE to configurelldb-dotest's shebang

[JDevlieghere]

Summary

Ideally we'd want all shebangs to be configurable, but that's not a viable solution. Given that lldb-dotest is already configured, we might as well make sure it uses the correct interpreter.

[JDevlieghere]

Context

[aprantl]

Yes, that's in the spirit of this script.

[labath]

I've been working around this by manually specifying the interpreter for a while now.... I didn't occur to me that the fix is that simple. :)

lldb/utils/lldb-dotest/lldb-dotest.in

-#!/usr/bin/env python
+#!@PYTHON_EXECUTABLE@
 import subprocess
 import sys