client: Fix task env calculation of alloc addr using static ports.

[jrasell]

The "NOMAD_ALLOC_ADDR_" task env var was not correctly generating the value when the allocation used a static port allocation. The resulting env var would always have an address with its port as 0.

The change correctly handles dynamic and static port definitions, so the "NOMAD_ALLOC_ADDR_" is properly formed.

Testing & Reproduction steps

The following job spec can be used on Linux to observe the behaviour:

job "example" {

  group "cache" {
    network {
      mode = "bridge"
      port "db" {
        static = 6379
      }
    }

    task "redis" {
      driver = "docker"

      config {
        image          = "redis:7"
        ports          = ["db"]
        auth_soft_fail = true
      }

      identity {
        env  = true
        file = true
      }

      resources {
        cpu    = 500
        memory = 256
      }
    }
  }
}

You can then use the command nomad alloc exec <alloc_id> env to query the task environment and identify the incorrect task env var.

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad product documentation, which is stored in the
  web-unified-docs repo. Refer to the web-unified-docs contributor guide for docs guidelines.
  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[pkazmierczak]

LGTM!