Allow chained references of custom environment variables to all generated envs

[siegenthalerroger]

Fixes #2227

Currently custom environment variables are added after most generated envs but notably before WAL_BUCKET_SCOPE_SUFFIX and _PREFIX. This means that referencing one of these values from a custom env is impossible (k8s has an implied ordering of env vars).

I have changed this to always have custom envs being at the end of the list while still respecting the overrideability of only certain envs. It is now possible to use WAL-E with Azure Storage Accounts while including the UID of the cluster in the backup path.

tldr: Works now, didn't before

apiVersion: v1
kind: ConfigMap
metadata:
  name: pod-env-overrides
  namespace: postgres-operator-system
data:
  WALG_AZ_PREFIX: "azure://container-name/$(SCOPE)$(WAL_BUCKET_SCOPE_SUFFIX)/$(PGVERSION)"