CI: the four actions at their latest releases, and Dependabot weekly

actions/checkout             v5.0.0 -> v7.0.1
  actions/upload-artifact      v6.0.0 -> v7.0.2
  actions/download-artifact    v4.3.0 -> v8.0.2
  docker/setup-buildx-action   v4.3.0 -> v4.4.1

Each stays pinned to a commit, with the version in its comment.

From the release notes, what the new majors change and why none of it
touches these workflows: download-artifact v5 moved the path of an
artifact fetched by ID, and ours is fetched by name; v8 fails on a
checksum mismatch where v4 said nothing, which is what a publish job
wants. checkout v7 refuses a fork's code under pull_request_target and
workflow_run, and the one workflow_run here checks nothing out. The
Node 24 releases need runner 2.327.1; ours run 2.337.0.

Pinned actions do not update themselves, so .github/dependabot.yml asks
for one grouped pull request a week.

.github/workflows/unit-test.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-24.04
     timeout-minutes: 10
     steps:
-      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
 
       # The tests run where the code they cover runs: on Arch, the same as
       # the live ISO and an Omarchy workstation. The container boots systemd