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:
https://github.com/NixOS/nix/blob/17e6b85d05b3d32df244b1d4e89aa41fd8bdcae8/src/nix-build/nix-build.cc#L516-L551
https://github.com/NixOS/nix/blob/db026103b18fb8b5a719594502edd0f89eb9c268/src/nix/get-env.sh

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

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