Flip order of transforms in init_ds_volumes_wf (#3238)

Closes #3237.

- Switch order of transforms in `boldref2target` node of `init_ds_volumes_wf` from boldref2anat_xfm --> anat2std_xfm to anat2std_xfm --> boldref2anat_xfm, as antsApplyTransforms expects transforms to be provided in reverse order.

Author: tsalo

fmriprep/workflows/bold/outputs.py

@@ -736,8 +736,10 @@ def init_ds_volumes_wf(
     workflow.connect([
         (inputnode, raw_sources, [('source_files', 'in_files')]),
         (inputnode, boldref2target, [
-            ('boldref2anat_xfm', 'in1'),
-            ('anat2std_xfm', 'in2'),
+            # Note that ANTs expects transforms in target-to-source order
+            # Reverse this for nitransforms-based resamplers
+            ('anat2std_xfm', 'in1'),
+            ('boldref2anat_xfm', 'in2'),
         ]),
         (inputnode, ds_bold, [
             ('source_files', 'source_file'),