installer: make sure we can always test the installer in ci and locally (backport #12059)

[mergify]

Just now there is a dependency on cachix, which means we cannot test the installer in CI if forks do not have the necessary secrets set up. We replace this with a simple http server that serves the installer and can be both used in CI and locally.

Another nice side effect is that now every pull request has a downloadable installer: https://github.com/NixOS/nix/actions/runs/12340760590

Motivation

Context

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The Nix maintainer team uses a GitHub project board to schedule and track reviews.

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This is an automatic backport of pull request #12059 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 6162e12 has failed:

On branch mergify/bp/2.25-maintenance/pr-12059
Your branch is up to date with 'origin/2.25-maintenance'.

You are currently cherry-picking commit 6162e1220.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   packaging/hydra.nix
	modified:   scripts/prepare-installer-for-github-actions
	new file:   scripts/serve-installer-for-github-actions

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/ci.yml
	both modified:   flake.nix

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally