run_unittests: use sys.executable on Windows for python

Instead of trying to guess whether we need py or python3, and then
falling over when whatever we guessed isn't in the path or isn't right,
just use sys.executable which should always work.

run_unittests.py

@@ -5926,10 +5926,7 @@ class NativeFileTests(BasePlatformTests):
         # invokes our python wrapper
         batfile = os.path.join(self.builddir, 'binary_wrapper{}.bat'.format(self.current_wrapper))
         with open(batfile, 'wt') as f:
-            if mesonbuild.environment.detect_msys2_arch():
-                f.write(r'@python3 {} %*'.format(filename))
-            else:
-                f.write('@{} {} %*'.format(sys.executable, filename))
+            f.write(r'@{} {} %*'.format(sys.executable, filename))
         return batfile
 
     def helper_for_compiler(self, lang, cb, for_machine = MachineChoice.HOST):