[lldb] fix -print-script-interpreter-info on windows

Apparently "{sys.prefix}/bin/python3" isn't where you find the
python interpreter on windows, so the test I wrote for
-print-script-interpreter-info is failing.

We can't rely on sys.executable at runtime, because that will point
to lldb.exe not python.exe.

We can't just record sys.executable from build time, because python
could have been moved to a different location.

But it should be OK to apply relative path from sys.prefix to sys.executable
from build-time to the sys.prefix at runtime.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D113650