use Ubuntu keyserver for CloudABI ports

The Ubuntu keyserver is more reliable than the MIT PGP server, which is
prone to going down. This commit also explicitly uses port 80 on the
keyserver for reasons outlined in rust-lang#57844.

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

@@ -32,9 +32,8 @@ ln -s ../lib/llvm-5.0/bin/lld /usr/bin/${target}-ld
 ln -s ../../${target} /usr/lib/llvm-5.0/${target}
 
 # Install the C++ runtime libraries from CloudABI Ports.
-echo deb https://nuxi.nl/distfiles/cloudabi-ports/debian/ cloudabi cloudabi > \
-    /etc/apt/sources.list.d/cloudabi.list
-curl 'https://pgp.mit.edu/pks/lookup?op=get&search=0x0DA51B8531344B15' | \
-    apt-key add -
+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'
+
 apt-get update
-apt-get install -y $(echo ${target} | sed -e s/_/-/g)-cxx-runtime
+apt-get install -y "${target//_/-}-cxx-runtime"