Can't shadow ARGS in Main

[fredrikekre]

Works in Julia < 1.5 but fails after:

$ julia14 -e 'ARGS=1'

$ julia15 -e 'ARGS=1'
ERROR: cannot assign a value to variable Base.ARGS from module Main

$ julia16 -e 'ARGS=1'
ERROR: cannot assign a value to variable Base.ARGS from module Main

$ julia-master -e 'ARGS=1'
ERROR: cannot assign a value to variable Base.ARGS from module Main

Similar to #34924 but that only happened with const.