cargo creusot rebuilds too much if any RUSTFLAGS argument changes

[Lysxia]

Issue first noticed here: #1212 (comment)

cargo creusot and cargo creusot --monolithic make each other rebuild everything including dependencies, even though the flag --monolithic should only affect the current package. The problem is that we are passing options in RUSTFLAGS which applies even to build dependencies. Previously (#1207) we had the opposite problem where cargo did not see the arguments that changed at all, so it did nothing at all when adding/removing --monolithic or any other creusot-rustc argument.

We need a more fine grained way than RUSTFLAGS to pass argument to creusot-rustc.

[xldenis]

Yea, changing RUSTFLAGS (correctly) causes cargo to rebuild everything. We definitely need to pass arguments through another mechanism, but then that re-introduces the stale detection issues with cargo. it might be worth asking on zulip / irlo.