Allow writing to default credential file for credential process mode

[faridnsh]

This brings back the older behaviour while updating the documentation to explicitly show the pitfall. More context on this issue.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 28.11%. Comparing base (99d6fe4) to head (820fba1).
Report is 150 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/saml2aws/commands/login.go	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1377       +/-   ##
===========================================
- Coverage   42.19%   28.11%   -14.09%     
===========================================
  Files          54       70       +16     
  Lines        6456    12536     +6080     
===========================================
+ Hits         2724     3524      +800     
- Misses       3283     8535     +5252     
- Partials      449      477       +28     

Flag	Coverage Δ
unittests	28.11% <0.00%> (-14.09%)	⬇️

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