[ sashite/nostamper.rs ] ci: force nightly for fuzz via RUSTUP_TOOLCHAIN
cargo +nightly fuzz failed to resolve the external subcommand on the
runner. Setting RUSTUP_TOOLCHAIN=nightly at job level overrides the
stable-pinned rust-toolchain.toml while keeping a bare `cargo fuzz`
invocation, which resolves cargo-fuzz correctly.
https://github.com/sashite/nostamper.rs/commit/e6cbdd437ac7626ccc2e4e99db659440290fe8de