[MotherSphere]

Drops the third-party dtolnay/rust-toolchain action from every workflow and installs Rust with rustup, which every GitHub-hosted runner image already carries. This follows Project-Colony-Resources templates/sign-and-publish-caller.yml and design/dependencies.md ("In CI, install Rust with rustup"): one less action pin to keep current, and that action publishes no versions Dependabot could move.

Changes

• release.yml, build job: the template's Install Rust step, verbatim (rustup toolchain install stable --profile minimal --no-self-update, rustup default stable, rustup target add ${{ matrix.target }}, shell: bash). Adding the target is a no-op on native legs and required on the Intel macOS leg. This was the only functional drift from the current template.
• ci.yml, check job (Linux, macOS, Windows): rustup toolchain install stable --profile minimal --component clippy,rustfmt --no-self-update then rustup default stable, with shell: bash so the same step runs on the Windows leg.
• ci.yml, MSRV job: rustup toolchain install "$RUSTUP_TOOLCHAIN" --profile minimal --no-self-update. RUSTUP_TOOLCHAIN still pins every cargo invocation, and the rustc --version guard still fails the job if the pin stops working. The comment no longer refers to the removed action.
• Swatinem/rust-cache stays in ci.yml only; the release build stays uncached.

No Rust code changes.

Verification

• Both workflow files parse as YAML; no reference to dtolnay/rust-toolchain remains in the repository.
• This PR's CI is the proof for ci.yml: fmt, clippy and tests on the 3 OS, plus the MSRV build.
• release.yml only runs on a release tag; its new step is the Resources template step, the same rustup commands Project-Colony-Resources runs in its own CI and release workflows.