build: adapt samples build configurations for monorepo (#13033)

Adapts samples build configurations for already-migrated libraries in
the monorepo by injecting the mandatory `BUILD_SUBDIR` environment
variable.

- Fixes `samples_build.yaml` files in already-migrated directories:
`java-bigquery`, `java-bigquerystorage`, `java-datastore`,
`java-logging`, and `java-storage`.

This ensures that samples build runs execute within their correct
subdirectories in the monorepo environment. Note that the automation
helper script and migration integration have been moved to the central
tooling branch.

Towards #12983

Author: meltsufin

java-bigquery/.cloudbuild/samples_build.yaml

@@ -20,6 +20,7 @@ steps:
   ]
   env:
   - 'JOB_TYPE=samples'
+  - 'BUILD_SUBDIR=java-bigquery'
   - 'BIGQUERY_PROJECT_ID=cloud-java-ci-sample'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
   - 'GCS_BUCKET=java-samples-bigquery'

java-bigquerystorage/.cloudbuild/samples_build.yaml

@@ -20,6 +20,7 @@ steps:
   ]
   env:
   - 'JOB_TYPE=samples'
+  - 'BUILD_SUBDIR=java-bigquerystorage'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
   - 'BIGTABLE_TESTING_INSTANCE=instance'
 - name: gcr.io/cloud-devrel-public-resources/java8

java-datastore/.cloudbuild/samples_build.yaml

@@ -20,6 +20,7 @@ steps:
   ]
   env:
   - 'JOB_TYPE=samples'
+  - 'BUILD_SUBDIR=java-datastore'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
   - 'KOKORO_GITHUB_PULL_REQUEST_NUMBER=$_PR_NUMBER'
 - name: gcr.io/cloud-devrel-public-resources/java11

java-logging/.cloudbuild/samples_build.yaml

@@ -19,6 +19,7 @@ steps:
     '.kokoro/build.sh'
   ]
   env:
+  - 'BUILD_SUBDIR=java-logging'
   - 'JOB_TYPE=samples'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
 - name: gcr.io/cloud-devrel-public-resources/java8

java-storage/.cloudbuild/samples_build.yaml

@@ -20,6 +20,7 @@ steps:
   ]
   env:
   - 'JOB_TYPE=samples'
+  - 'BUILD_SUBDIR=java-storage'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
   - 'GOOGLE_CLOUD_PROJECT_NUMBER=615621127317'
   - 'IT_SERVICE_ACCOUNT_EMAIL=samples@cloud-java-ci-sample.iam.gserviceaccount.com'