docs: fix devmode for manuals

[fricklerhandwerk]

Closes #362850

hard-coding a relative path will use the calling process's working
directory, but we want the path relative to the Nix expression.

this makes devmode in the following shells work again:

nix-shell doc

nix-shell nixos/doc/manual

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[fricklerhandwerk]

I'm really not sure what ofborg is trying to tell me here...

Cannot nix-instantiate `docs` because:
error: attribute 'docs' in selection path 'docs' not found
       Did you mean one of dcs, doas, aocd, boca or bochs?
Cannot nix-instantiate `docs.passthru.tests` because:
error: attribute 'docs' in selection path 'docs.passthru.tests' not found
       Did you mean one of dcs, doas, aocd, boca or bochs?

Where does it even get docs? I wasn't involved in anything like that here.

[ShamrockLee]

Thank you for being so patient. I'll test it tomorrow.

Where does it even get docs? I wasn't involved in anything like that here.

I guess it's the PR title. It attempted to build buildPython* and buildPython*.passthru.tests in my recent PRs.

[ShamrockLee]

The change looks reasonable.

Tested devmode command inside nix-shell doc, nix-shell nixos/doc/manual, nix develop -f doc/shell.nix, and nix develop -f doc/shell.nix, all working when CWD is the Nixpkgs project root. I also tried changing the CWD, and it also works.

The redirects command seems fixed before this PR.