Backport of [CI-only] Use the postinstall script for linux packages into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #13068 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

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

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Issue

Fixes #13067

Summary

The linux postinstall script was created as part of #12276 but wasn't actually used. This sets the reference so the script will actually be used.
The primary benefit is that the following lines are run automatically as part of the install instead of requiring users to create it themselves

mkdir -p /opt/nomad/data
chown nomad:nomad /opt/nomad/data
chown -R nomad:nomad /etc/nomad.d

[github-actions]

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