ci: Always retry the image build step

The image build step is prone to race conditions, e.g. token changes. It
also tends to hit sporadic connection errors to FDO's gitlab. To
minimize the risk of these types of issues block pipelines, always retry
the image building step if it failed.

This has the unwanted consequence that changes to the image building
that results in the script actually failing, but right now there doesn't
appear to be a way to distinguish between actual build errors, and the
mentioned race conditions, as both cause the script to fail.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2564>

.gitlab-ci.yml

@@ -115,6 +115,10 @@ variables:
         cd .. &&
         rm -rf virtme
       fi
+  retry:
+    max: 2
+    when:
+      - 'always'
 
 default:
   # Cancel jobs if newer commits are pushed to the branch