shell.nix: make wrapFactor helper idempotent
shell.nix: make wrapFactor helper idempotent

Detects if it was already wrapped, so it re-wraps correctly now.

tgunr/factor,tgunr/factor,factor/factor,mrjbq7/factor,tgunr/factor,factor/factor,mrjbq7/factor,tgunr/factor,mrjbq7/factor,factor/factor,tgunr/factor,mrjbq7/factor,mrjbq7/factor,factor/factor,mrjbq7/factor,factor/factor,factor/factor,tgunr/factor