docker: fix Valgrind errors on Xenial by updating to v3.12.0

The Valgrind version shipped with Xenial has some bugs that keep our
tests from working due to bad interactions with openssl [1]. Fix this by
using the "hola-launchpad/valgrind" PPA that provides a newer version
for which the bug has been fixed.

[1]: https://bugs.launchpad.net/ubuntu/+source/valgrind/+bug/1574437

azure-pipelines/docker/xenial

 ARG BASE
 FROM $BASE
-RUN apt-get update && \
+RUN echo 'deb http://ppa.launchpad.net/hola-launchpad/valgrind/ubuntu xenial main' >/etc/apt/sources.list.d/valgrind.list && \
+    apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8A0303A7544D59A08EBD1D621BCFD9273D292CF6 && \
+    apt-get update && \
     apt-get install -y --no-install-recommends \
         clang \
         cmake \