Allow overriding cargo dist's default runner #212

Job	Run time
cargo clippy -- -D warnings (ubuntu-latest)	8s
cargo fmt --all -- --check	4s
Code Coverage (ubuntu-latest, unix)	2m 15s
cargo check (ubuntu-latest)	7s
cargo clippy -- -D warnings (macOS-latest)	12s
Code Coverage (macos-latest, macos)	2m 44s
cargo check (macOS-latest)	13s
cargo clippy -- -D warnings (windows-latest)	1m 33s
Code Coverage (windows-latest, windows)	6m 2s
cargo check (windows-latest)	2m 1s
GNU test suite	38s
cargo test (ubuntu-latest)	44s
cargo test (macOS-latest)	2m 55s
cargo test (windows-latest)	3m 53s
	23m 29s

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