FIX: Serialize all interface arguments when exporting workflows

[effigies]

Summary

@matteomancini Apologies for taking over your PR. I had some maintenance changes to make, and ended up making more significant changes than I felt comfortable force-pushing to your branch. Feel free to take these changes to your PR, if you have further changes you want to make. I would also appreciate your review.

Fixes #3230.
Closes #3231.

List of changes proposed in this PR (pull-request)

#3231 Ensured that values passed to nodes would be reproduced after being serialized. This PR:

1. Added a test to verify that the problem was demonstrated and the solution resolved it. (It did.)
2. Rebased against maint/1.5.x so that this can go in the next release.
3. Decided to use f-strings and the repr (!r) format to avoid branching, then saw opportunities to further refactor.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3240 into maint/1.5.x will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           maint/1.5.x    #3240      +/-   ##
===============================================
+ Coverage        65.00%   65.03%   +0.02%     
===============================================
  Files              302      302              
  Lines            39942    39939       -3     
  Branches          5282     5280       -2     
===============================================
+ Hits             25964    25974      +10     
+ Misses           12918    12895      -23     
- Partials          1060     1070      +10     

Flag	Coverage Δ
#unittests	65.03% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/pipeline/engine/utils.py	74.48% <100.00%> (+2.87%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	65.68% <0.00%> (-3.44%)	⬇️
nipype/pipeline/plugins/base.py	58.08% <0.00%> (-1.65%)	⬇️

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