fix(validate-k8s-manifests): properly handle env & additional region values files

[ahoneiser]

The validate-k8s-manifests workflow does not respect region specific
values files as it should.

We support 3 "types" of naming patterns for values files at the moment:

• GPA
  Pattern: %domain%.values.(yaml|yml)
  e.g.: webservices.values.yaml

• tenant (env in primary region)
  Pattern: values(-ent)?-[[:alnum:]]+.(yaml|yml)
  e.g.: values-prod.yaml

• Tenant (env in additional region)
  Pattern: values(-ent)?-[[:alnum:]]+((-[[:alnum:]]+){2})?.(yaml|yml)
  e.g.: values-prod-europe-west1.yaml

We generally consider case 3 to be an additional values file that's
merged on top of case 2's values file (e.g. in ArgoCD). This change
renders case 3 to an extra artifact to make validation work as expected.

[grahamalama]

IIUC:

• The outer while loop looks at all values files that aren't values.{yml,yaml} (for chart types 1, 2, and 3)
• in that loop, we render charts for types 1 and 2
• in the inner while loop (while read region_values_file;), we render additional charts for the regional overrides (type 3)