var/loop/repeat: tweak FD 8 check and expr eval
var/loop/repeat: tweak FD 8 check and expr eval

lib/modernish/mdl/var/loop/repeat.mm:
- Improved check for ability to write to FD 8.
- Don't be so smart and just always evaluate the expression once
  in the main shell. Even if it's a no-op, it's negligible.

modernish/modernish