[CI] Swap synthstrip for bet when sloppy mode is chosen

[mattcieslak]

synthstrip is tough on the CI builds because it requires a lot of memory. This PR swaps in BET for synthstrip when the --sloppy flag is specified

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 23.21429% with 43 lines in your changes missing coverage. Please review.

Project coverage is 28.12%. Comparing base (634483f) to head (aedae87).

Files with missing lines	Patch %	Lines
qsiprep/interfaces/freesurfer.py	28.12%	23 Missing ⚠️
qsiprep/interfaces/anatomical.py	20.00%	8 Missing ⚠️
qsiprep/workflows/anatomical/volume.py	0.00%	6 Missing ⚠️
qsiprep/interfaces/images.py	33.33%	4 Missing ⚠️
qsiprep/workflows/dwi/hmc.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
- Coverage   28.13%   28.12%   -0.02%     
==========================================
  Files          69       69              
  Lines       10032    10079      +47     
  Branches     1298     1302       +4     
==========================================
+ Hits         2823     2835      +12     
- Misses       7111     7146      +35     
  Partials       98       98              

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