Use `rustup run <toolchain>` instead of `cargo +<toolchain>` #2568

Job	Run time
miri	1m 14s
test (1.26)	2m 31s
test (1.30)	2m 31s
test (1.31)	2m 30s
test (1.36)	2m 31s
test (1.39)	2m 28s
test (1.41)	2m 30s
test (stable)	2m 29s
test (beta)	2m 33s
test (nightly)	2m 33s
test (nightly, macos-11)	2m 31s
test (nightly, windows-2019)	2m 28s
build (aarch64-unknown-linux-gnu)	1m 32s
build (aarch64-unknown-linux-musl)	1m 31s
build (aarch64-apple-darwin)	2m 16s
build (aarch64-pc-windows-msvc)	2m 28s
build (x86_64-unknown-linux-gnu)	59s
build (x86_64-unknown-linux-musl)	1m 22s
build (x86_64-apple-darwin)	2m 1s
build (x86_64-pc-windows-msvc)	2m 26s
build (x86_64-unknown-freebsd)	2m 21s
deny / deny	24s
msrv / msrv	2m 24s
tidy / tidy	1m 3s
	49m 36s

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