Do not evaluate variables of string parameters in the nodes' description

Set the value of the string parameters in the nodes' description as
the unevaluated variable instead of the evaluated variable. The
evaluation will be made later, when the nodes with such parameters
will be computed.

meshroom/nodes/aliceVision/ImageMatchingMultiSfM.py

@@ -65,7 +65,7 @@ Thanks to this node, the FeatureMatching node will only compute the matches betw
             name='tree',
             label='Voc Tree: Tree',
             description='Input name for the vocabulary tree file.',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[],
             enabled=lambda node: 'VocabularyTree' in node.method.value,
         ),