Skip xargo installation for miri toolchain #1520

Job	Run time
Rust nightly on Linux	14s
Rust beta on Linux	13s
Rust stable on Linux	12s
Rust 1.62.0 on Linux	9s
Rust stable 18 months ago on Linux	10s
Rust stable minus 8 releases on Linux	8s
Rust nightly on macOS	28s
Rust beta on macOS	32s
Rust stable on macOS	30s
Rust 1.62.0 on macOS	18s
Rust stable 18 months ago on macOS	18s
Rust stable minus 8 releases on macOS	16s
Rust nightly on Windows	27s
Rust beta on Windows	30s
Rust stable on Windows	46s
Rust 1.62.0 on Windows	19s
Rust stable 18 months ago on Windows	20s
Rust stable minus 8 releases on Windows	20s
Rust 1.0.0 on Linux	4s
	6m 14s

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