"login oidc" CLI command sometimes skips printing auth URL for non-ttys

[cfryanr]

This PR is an attempt to fix the problem described in #1868 without introducing any new CLI flags.

Release note:

The `pinniped login oidc` command will skip printing the login URL to stderr if the has browser opened
and if stderr is not a tty. This is to avoid having the URL output on stdout confuse console UIs like k9s.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 38.14%. Comparing base (40e548e) to head (5bd73fc).

Files	Patch %	Lines
pkg/oidcclient/login.go	83.33%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1874   +/-   ##
=======================================
  Coverage   38.13%   38.14%           
=======================================
  Files         346      346           
  Lines       43628    43638   +10     
=======================================
+ Hits        16639    16647    +8     
  Misses      26474    26474           
- Partials      515      517    +2     

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[benjaminapetersen]

lgtm