nix/tests: run test help.sh only if nix is built with documentation (backport #11729)

[mergify]

Motivation

tests/functional/help.sh calls nix-* commands with option --help
if nix is built without documentation the option --help throws an error because the man page it wants to display is missing

Context

With disabled docs nix can not be built because of a failing test.

Priorities and Process

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This is an automatic backport of pull request #11729 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 85b0cd3 has failed:

On branch mergify/bp/2.18-maintenance/pr-11729
Your branch is up to date with 'origin/2.18-maintenance'.

You are currently cherry-picking commit 85b0cd320.
  (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/rm <file>..." as appropriate to mark resolution)
	both modified:   configure.ac
	deleted by us:   package.nix
	both modified:   tests/functional/local.mk

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