gitlab: add avocado asset caching

These can be quite big so lets cache them. I couldn't find any nots on
ccache in the gitlab docs so I've just ignored it for now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200701135652.1366-36-alex.bennee@linaro.org>

.gitlab-ci.yml

@@ -8,6 +8,12 @@ stages:
   - build
   - test
 
+# We assume GitLab has it's own caching set up for RPM/APT repositories so we
+# just take care of avocado assets here.
+cache:
+  paths:
+    - $HOME/avocado/data/cache
+
 include:
   - local: '/.gitlab-ci.d/edk2.yml'
   - local: '/.gitlab-ci.d/opensbi.yml'