fix(ci): add missing common::base64_encode to release-1.9 (#4588)

The cherry-pick of k8s cluster auth fix (c589180) included the call
to common::base64_encode in gke-helm.sh but did not include the function
definition in lib/common.sh, causing GKE Helm nightly tests to fail
with 'command not found' (exit 127).

Add the missing function and export statements from main.

Assisted-by: OpenCode

Author: zdrapela

.ci/pipelines/lib/common.sh

@@ -74,3 +74,12 @@ common::require_vars() {
     fi
   done
 }
+
+# Base64 encode a string (no newlines, cross-platform)
+common::base64_encode() {
+  echo -n "$1" | base64 | tr -d '\n'
+}
+
+# Export functions for subshell usage (e.g., timeout bash -c "...")
+export -f common::base64_encode
+export -f common::require_vars