[Toybox] One more bash corner case.

[Rob Landley]

In devuan deadmaus, which claims bash --version 5.2.15(1)-release 
(x86_64-pc-linux-gnu) when I do:

echo -e 'echo \n' | env -i PATH=${PATH at Q} PS1='\\$ ' bash --noediting 
--noprofile --norc -is

It prints $ three times instead of printing \$ three times. But when I 
add -c 'echo $PS1' it says "\\$" which has both slashes. (Which makes 
sense because single quotes, above...)

The man page says \\ is a literal backslash, but it's not printing.

What did I miss?

Rob