[FIX] Update FNIRT outputs for warped_file log_file to include cwd

[BenjaminMey]

Summary

Issue regarding the fix: #2899

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

• When warped_file or log_file input is set, will append the new name to the current working directory.

Acknowledgment

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

[codecov-io]

Codecov Report

Merging #2900 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2900      +/-   ##
==========================================
- Coverage   67.56%   67.54%   -0.03%     
==========================================
  Files         343      343              
  Lines       43601    43601              
  Branches     5427     5427              
==========================================
- Hits        29460    29451       -9     
- Misses      13436    13444       +8     
- Partials      705      706       +1

Flag	Coverage Δ
#smoketests	50.47% <0%> (ø)	⬆️
#unittests	64.96% <100%> (-0.04%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/fsl/preprocess.py	82.6% <100%> (ø)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	61.5% <0%> (-4.5%)	⬇️

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[mgxd]

LGTM, thanks!