ci: specify pull request environment variable for sample builds (#2178)

The build.sh has a logic to narrow the scope of the tests when it runs on a pull request. It uses an environmental variable.

# Previous Attempt

https://cloud.google.com/build/docs/optimize-builds/increase-vcpu-for-builds#increase_vcpu_for_default_pools
machineType: 'N1_HIGHCPU_8'

[The previous build](https://togithub.com/googleapis/java-bigtable/runs/22940962946) took 24 minutes. => It turned out this didn't help.

Author: suztomo

.cloudbuild/samples_build.yaml

@@ -22,6 +22,7 @@ steps:
   - 'JOB_TYPE=samples'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
   - 'BIGTABLE_TESTING_INSTANCE=instance'
+  - 'KOKORO_GITHUB_PULL_REQUEST_NUMBER=$_PR_NUMBER'
 - name: gcr.io/cloud-devrel-public-resources/java8
   entrypoint: echo
   args: [
@@ -30,5 +31,3 @@ steps:
 timeout: 3600s
 options:
   defaultLogsBucketBehavior: REGIONAL_USER_OWNED_BUCKET
-
-