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/CameraLocalization.py

@@ -125,7 +125,7 @@ class CameraLocalization(desc.CommandLineNode):
             name='voctree',
             label='Voctree',
             description='''[voctree] Filename for the vocabulary tree''',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[0],
             ),
         desc.File(