Ignore assignments to .KATI_READONLY

Since the rbc results file is sorted, .KATI_READONLY
ends up trying to make a variable that hasn't been defined
yet readonly, which is an error.

It's not really worth supporting moving it down in the list,
because it wouldn't affect the variable being writable while
still in Starlark code.

Bug: 226974242
Test: go test
Change-Id: I9402f69be97e5c7cf010ad86f124422ea55fda7f