[bugfix] Fix AttributeError crash in ssh backend

[vkarak]

Closes #3582.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.25%. Comparing base (1d35117) to head (7899a9f).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
reframe/core/schedulers/ssh.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3611      +/-   ##
==========================================
- Coverage   91.26%   91.25%   -0.02%     
==========================================
  Files          62       62              
  Lines       13583    13583              
==========================================
- Hits        12397    12395       -2     
- Misses       1186     1188       +2     

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