Prerelease install tests #311

Before merge, install the toolchains a manifest change touches on Linux and macOS, including the Cargo fallback for components with prebuilt artifacts. See sections 5 and 7 of this comment.

Changes:

• Diff the candidate manifest against the base to find the affected toolchains.
• Install and run the affected toolchains on x86_64-unknown-linux-gnu and aarch64-apple-darwin.
• Test the Cargo fallback on both targets.
• A fallback build doesn't count as passing a test that expects the prebuilt artifact.
• When the affected toolchains can't be determined, test all of them.

Existing PRs:

• tool: manifest diff #295: manifest diff.
• feat: run tests using diff #298: prerelease tests scoped to the toolchains a change touches.
• test: add pre-release checks on macos #299: prerelease on Linux and macOS, with the Cargo fallback.