[MotherSphere]

Replaces dtolnay/rust-toolchain with plain rustup, which every GitHub-hosted runner image already carries. This follows Project-Colony-Resources 7247b81: templates/sign-and-publish-caller.yml now installs Rust this way, and design/dependencies.md says CI should. One fewer third-party action to pin, and no master-commit pin that Dependabot cannot move.

What changed

• release.yml, build leg: the Install Rust step is now the template's step, verbatim (shell: bash, rustup toolchain install stable --profile minimal --no-self-update, rustup default stable, rustup target add ${{ matrix.target }}). Everything Spotter does differently from the template stays as it was: the TAG env, apt dependencies, --locked, asset names, colony.json validation, smoke tests, the Intel macOS architecture check and the Windows version-resource check. The sign-and-publish pin stays at 619460f.
• ci.yml, Check job: installs stable with --component clippy,rustfmt, shell: bash so the same step runs on the Linux, Windows and macOS legs.
• ci.yml, MSRV job: installs 1.90 the same way and makes it the default. Spotter has no rust-toolchain.toml, so nothing overrides it.

Verification

• The release step was diffed against the template: it matches byte for byte.
• Both workflows parse as YAML. No dtolnay reference is left in the repository.
• This PR's own CI runs the new Check (3 OS) and MSRV steps. The release build leg only runs on a release, and the next release-please merge exercises it. It runs the same commands the Check job runs here, plus rustup target add.