CI: examples: use stable Rust, not nightly

[W95Psp]

The CI builds (?), extracts with hax and verify the crates in the folder examples.
However, it cargo check with the ambiant Rust available on CI, which is not stable Rust, but the nightly hax pins.

To avoid breakages related to nightly VS stable (i.e. recently #1394), we should make the CI cargo check with a stable toolchain.