docs: add instructions for authenticating for system tests

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Source-Link: https://github.com/googleapis/synthtool/commit/363fe305e9ce34a6cd53951c6ee5f997094b54ee

[codecov[bot]]

Codecov Report

Merging #234 (54b93ba) into master (bf2749a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files           3        3           
  Lines         113      113           
  Branches       13       13           
=======================================
  Hits           90       90           
  Misses         23       23           

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