pubsub/azpubsub: Ensure 401s are not retryable

[vangent]

Fixes #3378

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4d69f8c) 77.49% compared to head (b363869) 77.48%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
pubsub/azuresb/azuresb.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3379      +/-   ##
==========================================
- Coverage   77.49%   77.48%   -0.02%     
==========================================
  Files         104      104              
  Lines       13968    13970       +2     
==========================================
  Hits        10824    10824              
- Misses       2380     2382       +2     
  Partials      764      764              

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