Backport of task env: add NOMAD_UNIX_ADDR var into release/1.10.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #25598 to be assessed for backporting due to the inclusion of the label backport/1.10.x.

The below text is copied from the body of the original PR.

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I keep thinking of this, and finally I'm doin it. Allows for easier usage in task setup:

task "nomad-cli" {
  driver = "raw_exec"
  config { ... run `nomad` commands ... }
  identity {
    env = true
  }
  env {
    NOMAD_ADDR = "${NOMAD_UNIX_ADDR}"
    # instead of this, which requires you to know the right "unix://" prefix, and socket location
    NOMAD_ADDR = "unix://${NOMAD_SECRETS_DIR}/api.sock"
  }
}

I named it "_UNIX_ADDR" instead of "_ADDR_UNIX" because I figure there may be services out there called "UNIX" that would cause a name collision, but I'm open to other names.

E2E spot check on my linux laptop:

    --- PASS: TestTaskAPI/testTaskAPI_NomadCLI (1.12s)

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Overview of commits

• 7519df8