Update golang-cli-helpers library, support env namespaces

What does this MR do?

Allows the namespace setting on our config entries to take affect for env entries, but only when an env name is prefixed with @.

This MR updates the library, but doesn't make use of the new functionality yet, as although the current env names can be verbose, there's little pointing in updating them.

Why was this MR needed?

Newer Kubernetes configuration settings would benefit from the new functionality: !3116 (merged)

An example of a namespace env can be found in the golang-cli-helper tests: https://gitlab.com/gitlab-org/golang-cli-helpers/-/blob/master/struct_field_test.go#L17

What's the best way to test this MR?

This changes nothing unless an env name is prefixed with @.

What are the relevant issue numbers?

#28236 (closed)