explicitly add cairo to make sure we can converge on a version that works with everything

containers/builder/spack.yaml

@@ -6,6 +6,7 @@ spack:
     - nlohmann-json
     - heppdt@3.04.01
     - clhep@2.4.1.3 cxxstd=17
+    - cairo+fc+ft+X+pdf+gobject
     - eigen@3.3.7
     - python@3.7.8
     - py-pip