ci_fairy: enable the 'do' keyword in Jinja2

From https://jinja.palletsprojects.com/en/2.11.x/extensions/ "The “do” aka expression-statement extension adds a simple do tag to the template engine that works like a variable expression but ignores the return value."

This is useful for list manipulation where we don't want to print 'None' when appending, e.g. {% do somelist.append([1, 2]) %}