Flux v2.9.1

This patch release focuses on hardening Flux's internal resource handling and fixing regressions. The primary improvement ensures that Flux's own CRDs are protected from accidental variable substitution during post-build processes, alongside important performance optimizations for Notation verification and various fixes for Kustomize and SOPS workflows.

Disable variable substitution on Flux CRDs — Prevents Flux from accidentally trying to substitute variables within its own Custom Resource Definitions (CRDs). Without this, users using post-build variable substitution could inadvertently corrupt their cluster's CRD schemas if those schemas contained sequences resembling variables.

Optimized Notation verification caching — Significantly reduces network overhead and improves verification speeds for users signing OCI artifacts with Notation, as registry tokens are now cached per verification instead of requested for every single check.

Restore in-memory Kustomization builds in CLI — Fixes a regression in the Flux CLI that could cause failures during in-memory kustomization builds, ensuring local validation and dry-runs remain reliable.