Support cross compiling to BSD and CI it (backport #8887)

[mergify]

Motivation

Fix #3280

Thanks to @alyssais effort, BSD cross compilation in Nixpkgs is now quite mature. Let's take advantage of this with Nix itself --- we've accepted patches to allow BSD builds before, but without CI bitrotting was quite likely. Now it won't bitrot, and people down download pre-built binaries.

Context

depends on #8569

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

Add 👍 to pull requests you find important.

---

This is an automatic backport of pull request #8887 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e44d2a6 has failed:

On branch mergify/bp/2.18-maintenance/pr-8887
Your branch is up to date with 'origin/2.18-maintenance'.

You are currently cherry-picking commit e44d2a6bb.
  (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)

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

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 564392b has failed:

On branch mergify/bp/2.18-maintenance/pr-8887
Your branch is ahead of 'origin/2.18-maintenance' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 564392b57.
  (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)

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

no changes added to commit (use "git add" and/or "git commit -a")

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

[Ericson2314]

We have in later versions, and that's good enough.