rustup download hangs in armv7 docker container running on an arm64 Linux

[djc]

As explained in rust-lang/rustup#3122.

rustup has two download backends, one based on curl and one based on reqwest:

https://github.com/rust-lang/rustup/blob/master/download/src/lib.rs

Minimal reproduction:

FROM arm32v7/ubuntu:16.04

RUN apt-get update && apt-get -y install curl

RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --verbose \
  --default-host armv7-unknown-linux-gnueabihf