fix(libtofs): “files_switch” mess up the variable defined by “map.jinja”

- “map.jinja” export a variable name as “<tplroot>”
- import of “libtofs.jinja” is done with context and has access to
  “<tplroot>” variable
- in “files_switch”, appending the empty string to the “fsl” variable
  modify globally “<tplroot>['tofs']['files_switch']”

* TEMPLATE/libtofs.jinja: do not use inplace “append” to avoid side effect.

TEMPLATE/libtofs.jinja

@@ -82,7 +82,7 @@
     ) %}
     {#- Append an empty value to evaluate as `default` in the loop below #}
     {%- if '' not in fsl %}
-      {%- do fsl.append('') %}
+      {%- set fsl = fsl + [''] %}
     {%- endif %}
     {%- for fs in fsl %}
       {%- for src_file in src_files %}