Add subcommand: `cargo msrv install-toolchain`

[Martijn Gribnau]

• Which resolves the MSRV via the usual route, i.e. --rust-version > package.rust-version in Cargo.toml > package.metadata.msrv
  • error: if no msrv defined
• Installs the toolchain with rustup
  • error: if rustup not installed
  • error: unable to install
  • ok: already present
  • ok: installed toolchain

Convenience command, to be used with github actions workflow.

Use case:

• In a workflow, install rustup without any other components
• Run cargo msrv install-toolchain
• Run whatever command next, e.g. cargo (opt. since only toolchain present: +) test or cargo msrv verify

Possible next steps:

• Add shorthands to test cargo msrv test [args...] runs cargo +<msrv> test [args...]
• similar: build, check, generate-lockfile ...