packaging: fix install path for loki.service & promtail.service

[ulidtko]

Small fix in Linux packaging.

There's a convention documented in man systemd.unit:

• /etc/systemd/system System units created by the administrator
• /usr/lib/systemd/system System units installed by the distribution package manager

Typically, if a package ships a systemd .service (especially in native apt/yum/dnf packages), the unit-file gets installed under /usr/lib/systemd/system/ — while the /etc location is reserved for downstream system vendor.

Grafana got this right:

$ dpkg -L grafana-enterprise | grep systemd/system/
/usr/lib/systemd/system/grafana-server.service

Notes for reviewer:

Doubtful if this is worth a mention in CHANGELOG or any further docs besides this PR.
Please suggest briefly in review if it is.

I also don't expect any breakage in upgrades, dpkg/rpm should handle this just fine.
(Although the separation of vendor/sysadmin VS packager responsibility can became a bitterly moot topic — especially when packages start reaching under /etc/systemd/system...)

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

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[ulidtko]

Hey @imiric, please review-approve?