fix(rez-build): use group for grouping and parser to set attributes

Use the group param to enable the grouping during the
display of the help menu. Use the parser param to correctly set the
attributes of the custom arguments that will become environment
variables.

Relates 992e7e9, #587

src/rezplugins/build_system/custom.py

@@ -60,7 +60,7 @@ def __init__(self, working_dir, opts=None, package=None, write_build_scripts=Fal
             child_build_args=child_build_args)
 
     @classmethod
-    def bind_cli(cls, parser):
+    def bind_cli(cls, parser, group):
         """
         Uses a 'parse_build_args.py' file to add options, if found.
         """
@@ -74,7 +74,7 @@ def bind_cli(cls, parser):
         before_args = set(x.dest for x in parser._actions)
 
         try:
-            exec source in {"parser": parser}
+            exec source in {"parser": group}
         except Exception as e:
             print_warning("Error in ./parse_build_args.py: %s" % str(e))