add NO_OP mode to rustup-init.sh #679

Job	Run time
build-linux-pr (dev, x86_64-unknown-linux-gnu)	9m 51s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	1m 43s
build-windows-master	0s
build-linux-pr (dev, aarch64-linux-android)	1m 2s
build-linux-master	0s
build-macos-x86_64 (dev, x86_64-apple-darwin)	14m 49s
build-linux-pr (release, x86_64-unknown-linux-gnu)	2m 27s
build-macos-x86_64 (release, x86_64-apple-darwin)	3m 0s
build-windows-pr (dev, x86_64-pc-windows-msvc)	8m 46s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	1m 48s
build-windows-stable	0s
build-windows-pr (release, x86_64-pc-windows-msvc)	4m 17s
build-linux-pr (release, aarch64-linux-android)	2m 50s
build-all-features (x86_64-unknown-linux-gnu)	4m 9s
build-freebsd-master	0s
build-freebsd-stable	0s
build-linux-stable	0s
build-macos-aarch64 (dev, aarch64-apple-darwin)	13m 33s
build-macos-aarch64 (release, aarch64-apple-darwin)	1m 45s
check	3m 17s
doc	5s
conclusion	1s
	1h 13m 23s

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