fixup! release: build unsigned Ubuntu .deb package

Currently, we target whatever GitHub Actions use as `ubuntu-latest`;
This, however, led to the unintentional requirement in v2.47.2.vfs.0.0
to run Ubuntu 24.04 (up from 22.04 in v2.47.1.vfs.0.1).

It is important to target a wider audience, though, especially in light
of CVE-2024-52005 which is only addressed in Git for Windows and
`microsoft/git`, but not Git.

We could now go back to 22.04; This would only be a temporary band-aid,
https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/
already announced that 20.04 is phased out very soon, and 22.04 will be
next.

Let's just use a Docker container instead that targets the oldest Ubuntu
LTS that is still maintained in _some_ shape or form.

We still verify in `validate-installers` that the resulting binary
installs and works on the latest Ubuntu version by virtue of using
`runs-on: ubuntu-latest` in _that_ matrix job.

Signed-off-by: Johannes Schindelin <[email protected]>

.github/workflows/build-git-installers.yml

@@ -493,6 +493,8 @@ jobs:
   # Build and sign Debian package
   create-linux-artifacts:
     runs-on: ubuntu-latest
+    container:
+      image: ubuntu:16.04 # expanded security maintenance until 04/02/2026, according to https://endoflife.date/ubuntu
     needs: prereqs
     environment: release
     steps: