[mlir] [Python] Fix misused lldb_build_directory in MLIR's test script (#122596)

Related to #121288 

This PR fixes the miscopied `config.lldb_build_directory` variable in
`lit.cfg.py` inside MLIR's test suit. `config.mlir_obj_root` is used as
a replacement for the copied python executable's directory.

**PS**: Since this is a common work-around on macOS, should we promote
it as a utility across projects?

Co-authored-by: Luohao Wang <Luohaothu@users.noreply.github.com>
Co-authored-by: Kai Sasaki <lewuathe@gmail.com>

Author: Luohaothu

mlir/test/lit.cfg.py

@@ -106,7 +106,7 @@ def find_real_python_interpreter():
     if sys.prefix != sys.base_prefix:
         copied_python = os.path.join(sys.prefix, "bin", "copied-python")
     else:
-        copied_python = os.path.join(config.lldb_build_directory, "copied-python")
+        copied_python = os.path.join(config.mlir_obj_root, "copied-python")
 
     # Avoid doing any work if we already copied the binary.
     if os.path.isfile(copied_python):