Relying on `.attrs.sh` to exist in `$NIX_BUILD_TOP` is problematic
because that's not compatible with how `nix-shell(1)` behaves. It places
`.attrs.{json,sh}` into a temporary directory and makes them accessible via
`$NIX_ATTRS_{SH,JSON}_FILE` in the environment[1]. The sole reason that
`nix-shell(1)` still works with structured-attrs enabled derivations
is that the contents of `.attrs.sh` are sourced into the
shell before sourcing `$stdenv/setup` (if `$stdenv` exists) by `nix-shell`.

However, the assumption that two files called `.attrs.sh` and
`.attrs.json` exist in `$NIX_BUILD_TOP` is wrong in an interactive shell
session and thus an inconsistency between shell debug session and actual
builds which can lead to unexpected problems.

To be precise, we currently have the following problem: an expression
like

  with import ./. {};
  runCommand "foo" { __structuredAttrs = true; foo.bar = [ 1 2 3 ]; }
    ''
      echo "''${__structuredAttrs@Q}"
      touch $out
    ''

prints `1` in its build-log. However when building interactively in a
`nix-shell`, it doesn't.

Because of that, I'm considering to propose a full deprecation of
`$NIX_BUILD_TOP/.attrs.{json,sh}`. A first step is to only mention the
environment variables, but not the actual paths anymore in Nix's
manual[2]. The second step - this patch - is to fix nixpkgs' stdenv
accordingly.

Please note that we cannot check for `-e "$NIX_ATTRS_JSON_FILE"` because
certain outdated Nix minors (that are still in the range of supported
Nix versions in `nixpkgs`) have a bug where `NIX_ATTRS_JSON_FILE` points
to the wrong file while building[3].

Also, for compatibility with Nix 2.3 which doesn't provide these
environment variables at all we still need to check for the existence of
.attrs.json/.attrs.sh here. As soon as we bump nixpkgs' minver to 2.4,
this can be dropped.

Finally, dropped the check for ATTRS_SH_FILE because that was never
relevant. In nix#4770 the ATTRS_SH_FILE variable was introduced[4] and
in a review iteration prefixed with NIX_[5]. In other words, these
variables were never part of a release and you'd only have this problem
if you'd use a Nix from a git revision of my branch from back then. In
other words, that's dead code.

[1] NixOS/nix#4770 (comment)
[2] NixOS/nix#9032
[3] NixOS/nix#6736
[4] NixOS/nix@3944a12
[5] NixOS/nix@27ce722

When specifying the `builder` attribute in `stdenv.mkDerivation`, this
will be effectively transformed into

    builtins.derivation {
      builder = stdenv.shell;
      args = [ "-e" builder ];
    }

This also means that `default-builder.sh` is never sourced and as a
result it's not guaranteed that `$NIX_ATTRS_SH_FILE` is set to a correct
location[1].

Also, we need to source `.attrs.sh` to source `$stdenv`. So, the
following is done now:

* If `$NIX_ATTRS_SH_FILE` points to a correct location, then use it.
  Directly using `.attrs.sh` is problematic for `nix-shell(1)` usage
  (see previous commit for more context), so prefer the environment
  variable if possible.

* Otherwise, if `.attrs.sh` exists, then use it. See [1] for when this
  can happen.

* If neither applies, it can be assumed that `__structuredAttrs` is
  turned off and thus nothing needs to be done.

[1] It's possible that it doesn't exist at all - in case of Nix 2.3 or
    it can point to a wrong location on older Nix versions with a bug in
    `__structuredAttrs`.