ci: prepend kokoro root directory to the path to service-acct.json

[stephaniewang526]

Fixes ERROR: (gcloud.auth.activate-service-account) Unable to read file [/tmpfs/src/gfile/service-acct.json]: [Errno 2] No such file or directory: '/tmpfs/src/gfile/service-acct.json'

Full trace: https://g3c.corp.google.com/results/invocations/a20c9009-fb3a-4f66-a6c1-11ad49a8db5b/targets/cloud-devrel%2Fclient-libraries%2Fjava%2Fjava-bigquery%2Fnightly%2Fsamples/log

[codecov]

Codecov Report

Merging #879 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #879   +/-   ##
=========================================
  Coverage     80.38%   80.38%           
  Complexity     1270     1270           
=========================================
  Files            79       79           
  Lines          6570     6570           
  Branches        755      755           
=========================================
  Hits           5281     5281           
  Misses          893      893           
  Partials        396      396           

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