out_stackdriver: correct env variable for SAs (#4753)
out_stackdriver: correct env variable for SAs (#4753)

Resolves #4617. It will also warn if the both
`GOOGLE_APPLCIATION_CREDENTIALS` and `GOOGLE_SERVICE_CREDENTIALS` are
being used.

Signed-off-by: Ridwan Sharif <6c5df95a21fafb35e738e7254e40f92703489cb1@google.com>
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