This fix will need to be backported to stable too, but we want the
testing images to be built on master, as other requirements
(specifically doc building) have changed.

It has been > 6 months since the release of Ubuntu 18.04 LTS.

Modify tutorial to remove warnings.

The old 'toolchain' package has been removed from the conda-forge
channel, and the 'gcc' package has no builds for Linux (several
non-authoratative sources say that it is "deprecated"), and it
seems that the toolchain3 package provides the necessary compilers