chore: Update release config to use keystore for rubygems key

.kokoro/release.cfg

@@ -32,11 +32,21 @@ env_vars: {
   value: "releasetool-publish-reporter-app,releasetool-publish-reporter-googleapis-installation,releasetool-publish-reporter-pem,docuploader_service_account"
 }
 
+# Pick up Rubygems key from internal keystore
+before_action {
+  fetch_keystore {
+    keystore_resource {
+      keystore_config_id: 73713
+      keyname: "rubygems-publish-key"
+      backend: "blade:keystore-fastconfigpush"
+    }
+  }
+}
 # Store the packages uploaded to rubygems.org, which
 # we can later use to generate SBOMs and attestations.
 action {
- define_artifacts {
-   regex: "github/common-protos-ruby/*/pkg/*.gem"
-   strip_prefix: "github"
- }
-}
+  define_artifacts {
+    regex: "github/common-protos-ruby/*/pkg/*.gem"
+    strip_prefix: "github"
+  }
+}

.trampolinerc

@@ -18,7 +18,13 @@ required_envvars+=(
 
 # Add env vars which are passed down into the container here.
 pass_down_envvars+=(
-    "AUTORELEASE_PR" "RELEASE_DRY_RUN" "RELEASE_PACKAGE" "KOKORO_GIT_COMMIT" "RUBY_VERSIONS" "EXTRA_CI_ARGS"
+    "AUTORELEASE_PR"
+    "EXTRA_CI_ARGS"
+    "KOKORO_GIT_COMMIT"
+    "KOKORO_KEYSTORE_DIR"
+    "RELEASE_DRY_RUN"
+    "RELEASE_PACKAGE"
+    "RUBY_VERSIONS"
 )
 
 # Prevent unintentional override on the default image.