No need to initialize the `release` environment for the `prereq` job;
Originally, the expectation was that this would prevent the repeated
need for validating manually that the jobs are allowed to access that
environment; However, GitHub Actions does ask for every single job, and
for certain reasons it is unlikely that this is ever going to be fixed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

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.

This requires a few modifications (`sudo` is not available, GitHub
Actions' node.js needs to be overridden, and we need to install a couple
of packages explicitly). In particular, we now need two jobs because it
turned out to be too convoluted to get `debsign` to work in a headless
workflow with Ubuntu 16.04; We still resort to `ubuntu-latest` for that
instead.

By still verifying the resulting binary in `validate-installers`, we
ensure that it 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 <johannes.schindelin@gmx.de>

Do use `apt-get` and ensure that it does not ask for confirmation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

By using cURL for `imap-send`, we can drop the libssl dependency, which
is not installed in Ubuntu 16.04 container by default.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This avoids some Bash-only `sed` invocations to strip quotes where no
stripping is required if `az` is asked to output the value in a more
appropriate format.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>