Guard getpass call

[ppinchuk]

Per the documentation of getpass.getuser, the call will throw an OSError on systems without access to the pwd module. This means importing rex can fail completely since we have a getpass.getuser call on import. In this PR, we delay the call to getpass.getuser until an HPC class instance is being created, allowing rex itself to be imported and used on systems missing pwd.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.85%. Comparing base (f9b0674) to head (471b1e9).

Files with missing lines	Patch %	Lines
rex/utilities/hpc.py	33.33%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (33.33%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   67.83%   67.85%   +0.01%     
==========================================
  Files          40       40              
  Lines        7114     7111       -3     
  Branches     1044     1043       -1     
==========================================
- Hits         4826     4825       -1     
+ Misses       1959     1958       -1     
+ Partials      329      328       -1     

Flag	Coverage Δ
unittests	67.85% <33.33%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
rex/utilities/hpc.py	48.91% <33.33%> (+0.16%)	⬆️

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