Revert "[libcxx][test] Use python specified by build rather than system default python"

This reverts commit 9f641c96cb.

The "python" command in gdb uses the python gdb is linked to,
not what "python" would give you if you used it directly in the shell.

libcxx/utils/libcxx/test/features.py

@@ -179,7 +179,7 @@ gdb.execute(\"quit\")"""
 
   try:
     stdout = subprocess.check_output(
-              [gdb_path, "-ex", sys.executable + " " + test_src, "--batch"],
+              [gdb_path, "-ex", "python " + test_src, "--batch"],
               stderr=subprocess.DEVNULL, universal_newlines=True)
   except subprocess.CalledProcessError:
     # We can't set breakpoint commands