software/theia: clear venv when installing python language server

Prevent this kind of errors when first installation fail:

    Error: [Errno 13] Permission denied: 'parts/python-language-server/bin/activate'

by using --clear the virtualenv is recreated from scratch if the command failed
for some reason.

software/theia/software.cfg

@@ -120,7 +120,7 @@ eggs =
 version = 0.19.0
 recipe = plone.recipe.command
 command =
-  PATH=${git:location}/bin/:$PATH  bash -c "${python3:executable} -m venv ${:location} && \
+  PATH=${git:location}/bin/:$PATH  bash -c "${python3:executable} -m venv --clear ${:location} && \
     . ${:location}/bin/activate && \
     pip install -r ${python-language-server-requirements.txt:output}"
 location = ${buildout:parts-directory}/${:_buildout_section_name_}