feat: allow version pinning & arbitrary downgrades via `RUSTUP_VERSION` #2247

Job	Run time
build-linux-master	0s
build-freebsd-master	0s
build-all-features (ubuntu-latest, x86_64-unknown-linux-gnu)	5m 9s
build-freebsd-stable	0s
build-all-features (windows-latest, x86_64-pc-windows-msvc)	9m 28s
build-linux-stable	0s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	5m 16s
build-macos-aarch64 (dev, aarch64-apple-darwin)	9m 34s
build-macos-x86_64 (dev, x86_64-apple-darwin)	8m 19s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	1m 7s
build-macos-aarch64 (release, aarch64-apple-darwin)	1m 45s
build-windows-master	0s
build-macos-x86_64 (release, x86_64-apple-darwin)	3m 59s
build-linux-pr (dev, aarch64-linux-android)	1m 15s
build-linux-pr (dev, aarch64-unknown-linux-gnu)	8m 31s
build-windows-stable	0s
build-windows-pr (dev, x86_64-pc-windows-msvc)	6m 24s
build-linux-pr (release, x86_64-unknown-linux-gnu)	2m 47s
check	50s
build-windows-pr (dev, aarch64-pc-windows-msvc)	2m 27s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	2m 22s
build-windows-pr (dev, x86_64-pc-windows-gnu)	2m 53s
build-linux-pr (release, aarch64-linux-android)	2m 22s
doc	7s
build-windows-pr (release, x86_64-pc-windows-msvc)	4m 17s
build-linux-pr (release, aarch64-unknown-linux-gnu)	3m 53s
build-windows-pr (release, aarch64-pc-windows-msvc)	5m 36s
build-windows-pr (release, x86_64-pc-windows-gnu)	5m 2s
conclusion	1s
	1h 33m 24s

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