ci: run every Rust job on the MSRV toolchain

[jo16oh]

test (stable) and test (1.98) run the same compiler, because the MSRV is the latest stable. Once a new stable ships, its clippy lints fail CI with no code change, and the required checks have no bypass.

• Run test once, on the MSRV
• Pin clippy, rustfmt and build-zed to the MSRV too, through one RUST_TOOLCHAIN in ci.yml
• Build the release binaries with the same RUST_TOOLCHAIN, set in release.yml
• Bump both with mise.toml and rust-version in the toolchain bump PR

The main-ci ruleset already requires test in place of test (stable) and test (1.98).