Add [EXE] substitution in snapshots

Author: weihanglo

tests/snapshots/init/unknown_flags.stderr

@@ -3,6 +3,6 @@ error: Found argument '--flag' which wasn't expected, or isn't valid in this con
 	If you tried to supply `--flag` as a value rather than a flag, use `-- --flag`
 
 USAGE:
-    cargo init <path>
+    cargo[EXE] init <path>
 
 For more information try --help