BaseTools: Update PYTHON env to PYTHON_COMMAND

Update PYTHON env to PYTHON_COMMAND.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Tests/PythonTest.py

@@ -0,0 +1,15 @@
+## @file
+# Test whether PYTHON_COMMAND is available
+#
+# Copyright (c) 2013 - 2018, Intel Corporation. All rights reserved.<BR>
+# This program and the accompanying materials
+# are licensed and made available under the terms and conditions of the BSD License
+# which accompanies this distribution.  The full text of the license may be found at
+# http://opensource.org/licenses/bsd-license.php
+#
+# THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+# WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+#
+
+if __name__ == '__main__':
+    print('TRUE')