build: pin the toolchain so a fresh clone can't fail on a stale default

`cargo xtask bundle` from a fresh clone failed with `rustc 1.94.0 is not
supported by the following package: xtask@0.10.1 requires rustc 1.97` when
the machine's `rustup default` was older than the workspace MSRV. The message
blames xtask only because `.cargo/config.toml` aliases `cargo xtask` to
`cargo run -p xtask`, so it is the first package in the build graph; cargo
rejects the whole workspace before anything compiles.

CI pins 1.97 but nothing provisioned it for builders. rust-toolchain.toml
pins the same version, with the clippy and rustfmt that `make lint` needs, so
rustup fetches it on the first build and local lints match the gate.

Verified from this commit: `cargo xtask --help` (the command that failed)
runs, `cargo xtask lint` is green, and `cargo xtask bundle` produces
target/bundle/disktree-0.10.1-aarch64-macos.zip.