Execute llvm-lit with the python found by CMake by default

The check-* targets run ${Python3_EXECUTABLE} $BUILD/bin/llvm-lit, but
running `./bin/llvm-lit $ARGS` from the build directory currently always
uses "python" to run llvm-lit. On most systems this will be python2.7 even
if we found python3 at CMake time.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D84625