Download private key from KeyStore

tools/internal_ci/linux/pull_request/grpc_microbenchmark_diff.cfg

@@ -17,6 +17,14 @@
 # Location of the continuous shell script in repository.
 build_file: "grpc/tools/internal_ci/linux/grpc_microbenchmark_diff.sh"
 timeout_mins: 120
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73836
+      keyname: "grpc_checks_private_key"
+    }
+  }
+}
 action {
   define_artifacts {
     regex: "**/*sponge_log.*"

tools/internal_ci/linux/pull_request/grpc_trickle_diff.cfg

@@ -17,6 +17,14 @@
 # Location of the continuous shell script in repository.
 build_file: "grpc/tools/internal_ci/linux/grpc_trickle_diff.sh"
 timeout_mins: 120
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73836
+      keyname: "grpc_checks_private_key"
+    }
+  }
+}
 action {
   define_artifacts {
     regex: "**/*sponge_log.*"

tools/internal_ci/macos/pull_request/grpc_ios_binary_size.cfg

@@ -18,6 +18,14 @@
 build_file: "grpc/tools/internal_ci/macos/grpc_ios_binary_size.sh"
 timeout_mins: 60
 gfile_resources: "/bigstore/grpc-testing-secrets/github_credentials/oauth_token.txt"
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73836
+      keyname: "grpc_checks_private_key"
+    }
+  }
+}
 action {
   define_artifacts {
     regex: "**/*sponge_log.*"

tools/run_tests/python_utils/check_on_pr.py

@@ -26,8 +26,8 @@ _GITHUB_REPO = 'grpc/grpc'
 _GITHUB_APP_ID = 22338
 _INSTALLATION_ID = 519109
 _GITHUB_APP_KEY = open(
-    os.environ['HOME'] + '/.ssh/google-grpc-checker.2018-12-13.private-key.pem',
+    os.path.join(os.environ['KOKORO_KEYSTORE_DIR'], '73836_grpc_checks_private_key'),
-    'r').read()
+    'rb').read()
 
 _ACCESS_TOKEN_CACHE = None