interpolate environment for services in script checks

[tgross]

Fixes #6836

In 0.10.2 (specifically #6586 / 387b016) we added interpolation to group service blocks and centralized the logic for task environment interpolation. This wasn't also added to script checks, which caused a regression where the IDs for script checks for services w/ interpolated fields (ex. the service name) didn't match the service ID that was registered with Consul.

This changeset calls the same taskenv interpolation logic during script_check configuration, and adds tests to reduce the risk of future regressions by comparing the IDs of service hook and the check hook.

---

In addition to the added unit test, I've exercised this with the following jobspec. If you run nomad job run example.nomad with this you'll see nginx-0 as the service name and the checks work, and if you update the value of PROJECT_VERSION that'll be properly reflected in the service name.

job "example" {
  datacenters = ["dc1"]

  group "webservers" {
    count = 1
    task "nginx" {
      driver = "docker"

      config {
        image = "0x74696d/nginx-with-curl:latest"
        port_map = {
          http = 80
        }
      }

      env {
        PROJECT_VERSION = "0"
      }

      service {
        name = "nginx-${PROJECT_VERSION}"
        port = "http"

        check {
          type     = "script"
          command = "/usr/bin/curl"
          args = ["--fail", "http://localhost/"]
          interval = "5s"
          timeout  = "3s"
        }
      }

      resources {
        memory = 128

        network {
          mbits = 10
          port "http" {}
        }
      }
    }
  }
}

cc @jorgemarey

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