Nightly is no longer necessary for wee_alloc, so we don't need to rely on unstable versions of Rust anymore.
As a bonus, this reduces size of each Rust-related Docker image by >1 GB:
- `squoosh-rotate`: 2.94GB -> 1.87GB
- `squoosh-resize`: 3.09GB -> 2.02GB
- `squoosh-hqx`: 3.13GB -> 2.06GB