Use workspace lints to control unexpected_cfgs. (#682)

By using `workspace.lints`, we can configure lints once for
everything (and then inherit it into the various crates).

The lint configuration for `unexpected_cfgs` works in 1.80+
and warns otherwise.

Co-authored-by: Thierry Berger <contact@thierryberger.com>

src_testbed/lib.rs

@@ -1,5 +1,4 @@
 #![allow(clippy::too_many_arguments)]
-#![allow(unexpected_cfgs)] // This happens due to the dim2/dim3/f32/f64 cfg.
 
 extern crate nalgebra as na;