add host_injections variant symlink

Author: bedroge

roles/create_cvmfs_content_structure/vars/dev.eessi.io.yml

@@ -12,4 +12,6 @@ files: # noqa: var-naming[no-role-prefix]
     dest: 'README'
     mode: '644'
 
-symlinks: [] # noqa: var-naming[no-role-prefix]
+symlinks: # noqa: var-naming[no-role-prefix]
+  host_injections: '$(EESSI_DEV_HOST_INJECTIONS:-/cvmfs/software.eessi.io/host_injections)'
+