zc.recipe.egg:custom traps on environment variables with embedded % signs

[Gottfried Ganßauge]

On my machine (OpenSuse-12.3) the system sets up two environment variables
"LESSOPEN=less_open.sh %s" and
"LESSCLOSE=lessclose.sh %s %s"

I have a buildout.cfg like follows:

[buildout]
parts = pytrs

[pytrs-env]
recipe = collective.recipe.environment

[pytrs]
recipe = zc.recipe.egg:custom
environment = pytrs-env
egg = haufe.pytrs

When run with bin/buildout install egg it generates the following backtrace:

(gotti@manta 1232) bin/buildout install pytrs
Installing pytrs.
While:
  Installing pytrs.

An internal error occurred due to a bug in either zc.buildout or in a
recipe being used:
Traceback (most recent call last):
  File "/home/gotti/source/zope-2.13/aurora-python.config/eggs/zc.buildout-1.5.2-py2.7.egg/zc/buildout/buildout.py", line 1805, in main
    getattr(buildout, command)(args)
  File "/home/gotti/source/zope-2.13/aurora-python.config/eggs/zc.buildout-1.5.2-py2.7.egg/zc/buildout/buildout.py", line 584, in install
    installed_files = self[part]._call(recipe.install)
  File "/home/gotti/source/zope-2.13/aurora-python.config/eggs/zc.buildout-1.5.2-py2.7.egg/zc/buildout/buildout.py", line 1297, in _call
    return f()
  File "/home/gotti/source/zope-2.13/aurora-python.config/eggs/zc.recipe.egg-2.0.1-py2.7.egg/zc/recipe/egg/custom.py", line 88, in install
    self._set_environment()
  File "/home/gotti/source/zope-2.13/aurora-python.config/eggs/zc.recipe.egg-2.0.1-py2.7.egg/zc/recipe/egg/custom.py", line 107, in _set_environment
    os.environ[key] = value % os.environ
TypeError: not enough arguments for format string
(gotti@manta 1233)

I think this should not kill the buildout but should instead print a warning an continue without further trying to interpolate anything on that specific value.