chore: set env vars for tests and make it executable

[anguillanneuf]

This should fix ERROR: (gcloud.auth.activate-service-account) in tests.

Java 11 samples tests passed but Java 8 kept failing with this error:

[16:22:51][ERROR] Failed to get build config
com.google.devtools.kokoro.config.ConfigException: Couldn't find build configuration java8-samples under /tmp/workspace/workspace/cloud-devrel/client-libraries/java/java-pubsub/presubmit/java8-samples/src/github/java-pubsub/.kokoro/presubmit.

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I had a hard-to-spot typo in jave8-samples.cfg. It should have been java8-samples.cfg.

[stephaniewang526]

looks like Java 8 samples is still failing

[codecov]

Codecov Report

Merging #204 into master will decrease coverage by 0.51%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #204      +/-   ##
============================================
- Coverage     78.88%   78.37%   -0.52%     
  Complexity      302      302              
============================================
  Files            21       21              
  Lines          2719     2719              
  Branches        150      150              
============================================
- Hits           2145     2131      -14     
  Misses          513      513              
- Partials         61       75      +14     

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsub/v1/SubscriptionAdminClient.java	57.47% <0.00%> (-5.15%)	49.00% <0.00%> (ø%)
...a/com/google/cloud/pubsub/v1/TopicAdminClient.java	52.05% <0.00%> (-2.06%)	29.00% <0.00%> (ø%)

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