jinja2: Fix zc.buildout related import

jinja2_template imports zc.buildout, but uses zc.buildout.buildout (note the double .buildout). As the result, when tried to use in standalone mode, importing slapos.recipe.template.jinja2_template fails:

In [1]: import slapos.recipe.template.jinja2_template
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[1], line 1
----> 1 import slapos.recipe.template.jinja2_template

File ~/src/wendelin/slapos/slapos.recipe.template/slapos/recipe/template/jinja2_template.py:56
     53     raise AssertionError(*args)
     54 DEFAULT_CONTEXT['assert'] = _assert
---> 56 _buildout_safe_dumps = getattr(zc.buildout.buildout, 'dumps', None)
     57 DUMPS_KEY = 'dumps'
     58 DEFAULT_IMPORT_DELIMITER = '/'

AttributeError: module 'zc.buildout' has no attribute 'buildout'

That happens because zc.buildout is just a package and does not import anything from its submodules:

https://lab.nexedi.com/nexedi/slapos.buildout/blob/e9c39301/src/zc/buildout/init.py

So the correct way of importing is, thus, import zc.buildout.buildout.

-> Fix it.

The bug was there since 65cd02ec (Add jinja2-based template recipe entry.)

/cc @vpelletier, @jerome