This change modifies appendEnvVars() to allow environment variables
defined in the PostgreSQL CRD spec.env to override operator-generated
environment variables (like SPILO_CONFIGURATION) instead of being
silently ignored.
Previously, if an env var already existed in the list, user-provided
values were skipped. Now, user values take precedence and replace
the operator-generated ones.
This enables users to customize SPILO_CONFIGURATION and other
operator-managed env vars through the CRD, which is useful for
adding custom Patroni DCS configuration like ignore_slots.
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