tools/makemanifest.py: Use sys.executable when invoking Python scripts.

So the version of Python used to run makemanifest.py is also used for the
sub-scripts.

tools/makemanifest.py

@@ -270,13 +270,13 @@ def main():
         return
 
     # Freeze paths as strings
-    res, output_str = system([MAKE_FROZEN] + str_paths)
+    res, output_str = system([sys.executable, MAKE_FROZEN] + str_paths)
     if res != 0:
         print('error freezing strings {}: {}'.format(str_paths, output_str))
         sys.exit(1)
 
     # Freeze .mpy files
-    res, output_mpy = system([MPY_TOOL, '-f', '-q', args.build_dir + '/genhdr/qstrdefs.preprocessed.h'] + mpy_files)
+    res, output_mpy = system([sys.executable, MPY_TOOL, '-f', '-q', args.build_dir + '/genhdr/qstrdefs.preprocessed.h'] + mpy_files)
     if res != 0:
         print('error freezing mpy {}: {}'.format(mpy_files, output_mpy))
         sys.exit(1)