Fix environment constructing in MSYS2 shell.

Author: Blackhex

emsdk.py

@@ -129,7 +129,7 @@ def exit_with_error(msg):
   else:
     BASH = True
 
-if WINDOWS:
+if WINDOWS and not MSYS:
   ENVPATH_SEPARATOR = ';'
 else:
   ENVPATH_SEPARATOR = ':'