Add simple retry to rustup commands in `rb_sys/mkmf` (#518) #479

Job	Run time
📥 Fetch CI data	1s
docker_images (arm-linux, arm-unknown-linux-gnueabihf, docker/Dockerfile.arm-linux, arm-linux-gnu...	2m 40s
docker_images (aarch64-linux, aarch64-linux-gnu, aarch64-unknown-linux-gnu, aarch64-unknown-linux...	2m 49s
docker_images (aarch64-linux-musl, aarch64-unknown-linux-musl, docker/Dockerfile.aarch64-linux-mu...	2m 50s
docker_images (arm64-darwin, aarch64-apple-darwin, docker/Dockerfile.arm64-darwin, aarch64-apple-...	2m 55s
docker_images (x64-mingw-ucrt, x86_64-pc-windows-gnu, docker/Dockerfile.x64-mingw-ucrt, x86_64-wi...	2m 51s
docker_images (x64-mingw32, x86_64-pc-windows-gnu, docker/Dockerfile.x64-mingw32, x86_64-windows-...	2m 38s
docker_images (x86_64-darwin, x86_64-apple-darwin, docker/Dockerfile.x86_64-darwin, x86_64-apple-...	2m 47s
docker_images (x86_64-linux, x86_64-linux-gnu, x86_64-unknown-linux-gnu, x86_64-unknown-linux-gnu...	2m 52s
docker_images (x86_64-linux-musl, x86_64-unknown-linux-musl, docker/Dockerfile.x86_64-linux-musl,...	3m 17s
	25m 40s

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