CI/tools: Add fuse2fs dependency for singularity installation

[adswa]

Should provide a fix for #263

[adswa]

The singularity installation succeeds now.
There is a test failure, but I don't think it needs addressing in this PR:

============================================================================================================
FAILED ../adapters/tests/test_docker.py::test_load_multi_image - datalad.runner.exception.CommandError: CommandError: 'docker pull alpine:3.15' failed with exitcode 1
======================================================================================= 1 failed, 28 passed, 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (49d30bc) 94.64% compared to head (52eefbd) 94.64%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #264   +/-   ##
=======================================
  Coverage   94.64%   94.64%           
=======================================
  Files          24       24           
  Lines        1120     1120           
=======================================
  Hits         1060     1060           
  Misses         60       60           

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[yarikoptic]

I restarted that appveyor run -- apparently was some intermittent failure, so we are all good. I do not think we need a release for it but I could be proven to be wrong ;)