credentials/google: Add cloud-platform scope for ADC

[halvards]

Fixes #7886

RELEASE NOTES:

• credentials/google: set scope for application default credentials

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.14%. Comparing base (4c07bca) to head (35e8dd5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7887      +/-   ##
==========================================
+ Coverage   81.84%   82.14%   +0.29%     
==========================================
  Files         377      377              
  Lines       38120    38120              
==========================================
+ Hits        31201    31312     +111     
+ Misses       5603     5518      -85     
+ Partials     1316     1290      -26     

Files with missing lines	Coverage Δ
credentials/google/google.go	77.33% <100.00%> (ø)

... and 22 files with indirect coverage changes

[dfawley]

Thanks for the fix!