chore(python): use BUILD_SPECIFIC_GCLOUD_PROJECT for samples

https://github.com/googleapis/python-talent/blob/ef045e8eb348db36d7a2a611e6f26b11530d273b/samples/snippets/noxfile_config.py#L27-L32

`BUILD_SPECIFIC_GCLOUD_PROJECT` is an alternate project used for sample tests that do poorly with concurrent runs on the same project.

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Wed Sep 30 13:06:03 2020 -0600
Source-Repo: googleapis/synthtool
Source-Sha: 9b0da5204ab90bcc36f8cd4e5689eff1a54cc3e4
Source-Link: googleapis/synthtool@9b0da52

Author: yoshi-automation

.kokoro/samples/python3.6/common.cfg

@@ -13,6 +13,12 @@ env_vars: {
     value: "py-3.6"
 }
 
+# Declare build specific Cloud project.
+env_vars: {
+    key: "BUILD_SPECIFIC_GCLOUD_PROJECT"
+    value: "python-docs-samples-tests-py36"
+}
+
 env_vars: {
     key: "TRAMPOLINE_BUILD_FILE"
     value: "github/python-bigquery-datatransfer/.kokoro/test-samples.sh"

.kokoro/samples/python3.7/common.cfg

@@ -13,6 +13,12 @@ env_vars: {
     value: "py-3.7"
 }
 
+# Declare build specific Cloud project.
+env_vars: {
+    key: "BUILD_SPECIFIC_GCLOUD_PROJECT"
+    value: "python-docs-samples-tests-py37"
+}
+
 env_vars: {
     key: "TRAMPOLINE_BUILD_FILE"
     value: "github/python-bigquery-datatransfer/.kokoro/test-samples.sh"

.kokoro/samples/python3.8/common.cfg

@@ -13,6 +13,12 @@ env_vars: {
     value: "py-3.8"
 }
 
+# Declare build specific Cloud project.
+env_vars: {
+    key: "BUILD_SPECIFIC_GCLOUD_PROJECT"
+    value: "python-docs-samples-tests-py38"
+}
+
 env_vars: {
     key: "TRAMPOLINE_BUILD_FILE"
     value: "github/python-bigquery-datatransfer/.kokoro/test-samples.sh"

synth.metadata

@@ -19,14 +19,14 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "da29da32b3a988457b49ae290112b74f14b713cc"
+        "sha": "9b0da5204ab90bcc36f8cd4e5689eff1a54cc3e4"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "da29da32b3a988457b49ae290112b74f14b713cc"
+        "sha": "9b0da5204ab90bcc36f8cd4e5689eff1a54cc3e4"
       }
     }
   ],