Use `rustup run <toolchain>` instead of `cargo +<toolchain>` #2571

Job	Run time
miri	1m 30s
test (stable)	8m 1s
test (beta)	6m 58s
test (nightly)	7m 28s
test (nightly, macos-11)	12m 11s
test (nightly, windows-2019)	11m 8s
build (aarch64-unknown-linux-gnu)	2m 12s
build (aarch64-unknown-linux-musl)	1m 33s
build (aarch64-apple-darwin)	3m 5s
build (aarch64-pc-windows-msvc)	3m 51s
build (x86_64-unknown-linux-gnu)	1m 14s
build (x86_64-unknown-linux-musl)	1m 34s
build (x86_64-apple-darwin)	2m 17s
build (x86_64-pc-windows-msvc)	3m 53s
build (x86_64-unknown-freebsd)	3m 12s
test (1.26)	12m 26s
test (1.30)	9m 54s
test (1.31)	10m 33s
test (1.36)	12m 3s
test (1.39)	10m 2s
test (1.41)	9m 54s
deny / deny	22s
msrv / msrv	2m 29s
tidy / tidy	1m 9s
	2h 18m 59s

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