fetch cloudabi packages from our own mirror

The upstream archive went down due to an expired certificate, so I
mirrored the whole thing on our mirrors S3 bucket. This is safe, as the
certificate just seemed to be expired and the contents of the APT
archive are signed anyway.

src/ci/docker/dist-various-2/build-cloudabi-toolchain.sh

@@ -33,7 +33,8 @@ ln -s ../../${target} /usr/lib/llvm-5.0/${target}
 
 # Install the C++ runtime libraries from CloudABI Ports.
 apt-key adv --batch --yes --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 0DA51B8531344B15
-add-apt-repository -y 'deb https://nuxi.nl/distfiles/cloudabi-ports/debian/ cloudabi cloudabi'
+add-apt-repository -y \
+    'deb https://ci-mirrors.rust-lang.org/rustc/2020-05-04-cloudabi-apt-archive/ cloudabi cloudabi'
 
 apt-get update
 apt-get install -y "${target//_/-}-cxx-runtime"