Allow user env vars to override operator-generated ones

[PavelZaytsev]

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.

Testing

Tested with:

• PostgreSQL 17 cluster with custom SPILO_CONFIGURATION
• Verified that user-provided env vars properly override operator-generated ones
• Confirmed Patroni configuration contains both operator defaults and user customizations
• Validated that ignore_slots configuration works correctly with native PostgreSQL 17 slot synchronization

Example working configuration:

apiVersion: "acid.zalan.do/v1"
kind: postgresql
spec:
  env:
    - name: SPILO_CONFIGURATION
      value: |
        bootstrap:
          dcs:
            ignore_slots:
              - type: logical

Result: Patroni config includes both operator settings (ttl, loop_wait, etc.) and user's ignore_slots configuration.

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