feat: allow version pinning & arbitrary downgrades via `RUSTUP_VERSION` #2249

Job	Run time
build-freebsd-stable	0s
build-all-features (ubuntu-latest, x86_64-unknown-linux-gnu)	5m 19s
build-freebsd-master	0s
build-linux-master	0s
build-all-features (windows-latest, x86_64-pc-windows-msvc)	10m 22s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	5m 20s
build-macos-x86_64 (dev, x86_64-apple-darwin)	8m 22s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	1m 8s
build-macos-aarch64 (dev, aarch64-apple-darwin)	9m 17s
build-linux-stable	0s
build-macos-x86_64 (release, x86_64-apple-darwin)	5m 1s
build-linux-pr (dev, aarch64-linux-android)	1m 18s
build-macos-aarch64 (release, aarch64-apple-darwin)	2m 0s
build-linux-pr (dev, aarch64-unknown-linux-gnu)	7m 33s
build-linux-pr (release, x86_64-unknown-linux-gnu)	2m 36s
build-windows-master	0s
build-windows-pr (dev, x86_64-pc-windows-msvc)	7m 37s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	2m 16s
build-windows-stable	0s
build-linux-pr (release, aarch64-linux-android)	2m 44s
build-windows-pr (dev, aarch64-pc-windows-msvc)	3m 54s
build-linux-pr (release, aarch64-unknown-linux-gnu)	5m 25s
check	41s
build-windows-pr (dev, x86_64-pc-windows-gnu)	3m 42s
build-windows-pr (release, x86_64-pc-windows-msvc)	4m 23s
doc	5s
build-windows-pr (release, aarch64-pc-windows-msvc)	4m 1s
build-windows-pr (release, x86_64-pc-windows-gnu)	4m 54s
conclusion	1s
	1h 37m 59s

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