doc/stdenv: rewrite manual build procedure to be closer to an auto-build (#262137)

* doc/stdenv: rewrite manual build procedure to be closer to an auto-build

This is based on
<https://jade.fyi/blog/building-nix-derivations-manually/> plus some
more original research.

The previous version of this section did not work for your choice of
simple Haskell package, e.g. haskellPackages.hscolour, due to things
like `compileBuildDriverPhase` and other custom phases that it
does not address at all.

It seems more correct to use genericBuild in development to harmonize it
with what is actually done.

I feel a little bit like I am committing a sin by suggesting using the
experimental CLI in the manual (afaict I am the first to do this), but I
have given the old version of the command, and there are justifiable
reasons to do it:
* The noted limitations with env-vars are fixed. The one with the
  non-empty temp directory was one I ran into myself and oh boy was that
  not fun to debug.
* Additionally the outputs are set *before* sourcing `setup.sh`: there
  is an issue with nix-shell where the original version of `$out` winds
  up in `NIX_LDFLAGS` due to _addRpathPrefix, which means that resulting
  executables may not run properly.

It is sad that `nix develop` propagates a wrong value of `SHELL` to
builders, though. It is equally sad that `nix-shell` is essentially
abandoned upstream, with undocumented and not insignificant differences
from `nix develop`.

For the exact script differences:
17e6b85d05/src/nix-build/nix-build.cc (L516-L551)
db026103b1/src/nix/get-env.sh

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/stdenv/stdenv.chapter.md

@@ -101,25 +101,62 @@ genericBuild
 
 ### Building a `stdenv` package in `nix-shell` {#sec-building-stdenv-package-in-nix-shell}
 
-To build a `stdenv` package in a [`nix-shell`](https://nixos.org/manual/nix/unstable/command-ref/nix-shell.html), use
+To build a `stdenv` package in a [`nix-shell`](https://nixos.org/manual/nix/unstable/command-ref/nix-shell.html), enter a shell, find the [phases](#sec-stdenv-phases) you wish to build, then invoke `genericBuild` manually:
+
+Go to an empty directory, invoke `nix-shell` with the desired package, and from inside the shell, set the output variables to a writable directory:
 
 ```bash
+cd "$(mktemp -d)"
 nix-shell '<nixpkgs>' -A some_package
-eval "${unpackPhase:-unpackPhase}"
-cd $sourceRoot
-eval "${patchPhase:-patchPhase}"
-eval "${configurePhase:-configurePhase}"
-eval "${buildPhase:-buildPhase}"
+export out=$(pwd)/out
+```
+
+Next, invoke the desired parts of the build.
+First, run the phases that generate a working copy of the sources, which will change directory to the sources for you:
+
+```bash
+phases="${prePhases[*]:-} unpackPhase patchPhase" genericBuild
+```
+
+Then, run more phases up until the failure is reached.
+For example, if the failure is in the build phase, the following phases would be required:
+
+```bash
+phases="${preConfigurePhases[*]:-} configurePhase ${preBuildPhases[*]:-} buildPhase" genericBuild
+```
+
+Re-run a single phase as many times as necessary to examine the failure like so:
+
+```bash
+phases="buildPhase" genericBuild
 ```
 
 To modify a [phase](#sec-stdenv-phases), first print it with
 
+```bash
+echo "$buildPhase"
+```
+
+Or, if that is empty, for instance, if it is using a function:
+
 ```bash
 type buildPhase
 ```
 
 then change it in a text editor, and paste it back to the terminal.
 
+::: {.note}
+This method may have some inconsistencies in environment variables and behaviour compared to a normal build within the [Nix build sandbox](https://nixos.org/manual/nix/unstable/language/derivations#builder-execution).
+The following is a non-exhaustive list of such differences:
+
+- `TMP`, `TMPDIR`, and similar variables likely point to non-empty directories that the build might conflict with files in.
+- Output store paths are not writable, so the variables for outputs need to be overridden to writable paths.
+- Other environment variables may be inconsistent with a `nix-build` either due to `nix-shell`'s initialization script or due to the use of `nix-shell` without the `--pure` option.
+
+If the build fails differently inside the shell than in the sandbox, consider using [`breakpointHook`](#breakpointhook) and invoking `nix-build` instead.
+The [`--keep-failed`](https://nixos.org/manual/nix/unstable/command-ref/conf-file#opt--keep-failed) option for `nix-build` may also be useful to examine the build directory of a failed build.
+:::
+
 ## Tools provided by `stdenv` {#sec-tools-of-stdenv}
 
 The standard environment provides the following packages: