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/KeyframeSelection.py

@@ -95,14 +95,14 @@ You can extract frames at regular interval by configuring only the min/maxFrameS
             name='sensorDbPath',
             label='Sensor Db Path',
             description='''Camera sensor width database path.''',
-            value=os.environ.get('ALICEVISION_SENSOR_DB', ''),
+            value='${ALICEVISION_SENSOR_DB}',
             uid=[0],
         ),
         desc.File(
             name='voctreePath',
             label='Voctree Path',
             description='''Vocabulary tree path.''',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[0],
         ),
         desc.BoolParam(