Introduce `RUSTUP_CONCURRENT_DOWNLOADS` to control concurrency #2950

Job	Run time
build-windows-pr (release, x86_64-pc-windows-msvc)	5m 5s
build-windows-pr (dev, x86_64-pc-windows-msvc)	6m 49s
build-windows-pr (release, aarch64-pc-windows-msvc)	4m 3s
build-windows-pr (dev, x86_64-pc-windows-gnu)	2m 28s
build-windows-pr (release, x86_64-pc-windows-gnu)	4m 10s
build-windows-pr (dev, aarch64-pc-windows-msvc)	12m 7s
build-linux-pr (dev, aarch64-unknown-linux-gnu)	7m 26s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	5m 21s
build-windows-stable	0s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	1m 15s
build-linux-pr (release, x86_64-unknown-linux-gnu)	2m 54s
build-linux-pr (dev, aarch64-linux-android)	1m 17s
build-all-features (ubuntu-latest, x86_64-unknown-linux-gnu)	5m 30s
doc	5s
build-linux-pr (release, aarch64-linux-android)	2m 50s
build-windows-master	0s
build-linux-pr (release, aarch64-unknown-linux-gnu)	3m 47s
check	58s
build-freebsd-master	0s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	2m 34s
build-all-features (windows-latest, x86_64-pc-windows-msvc)	9m 47s
build-linux-master	0s
build-macos-x86_64 (release, x86_64-apple-darwin)	3m 59s
build-macos-x86_64 (dev, x86_64-apple-darwin)	19m 53s
build-macos-aarch64 (dev, aarch64-apple-darwin)	9m 38s
build-freebsd-stable	0s
build-macos-aarch64 (release, aarch64-apple-darwin)	2m 14s
build-linux-stable	0s
conclusion	3s
	1h 54m 13s

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