fix(build): Use nix from the buildir instead of the installed one

[Et7f3]

Motivation

Context

fix #7124

I think it wasn't caught because nix develop configurePhase set a prefix to local directory that is writeable.

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
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes

[Et7f3]

It can be made more generic with

Suggested change

	nix_PATH = $(buildprefix)src/nix/nix
	nix_PATH = $(buildprefix)$(nix_DIR)/nix

$(nix_NAME) isn't available without .SECONDEXPANSION:

[edolstra]

Our build system by intent uses make install, not make. (E.g. the main tests are make installcheck.) We should probably just remove the rules that build $(buildprefix)src/nix/nix etc. to avoid confusion.

[Et7f3]

So we will need in configure to give a prefix writable by user ? I think it is more common and less disturbing to have a CMMI (without needing to pass --prefix) and make that is enough for on root users (without having to use a prefix)

[fricklerhandwerk]

Triaged in the Nix team meeting 2023-02-17:

• @edolstra: probably a misunderstanding of the build process. one should use make install. this may be surprising for people used to other build processes.
• to discuss how to solve the underlying problem

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-02-17-nix-team-meeting-minutes-33/25624/1

[Ericson2314]

Duplicate of #5145. I changed the include order to avoid the expansion order problems and avoid needing to define twice.