stdenv/setup.sh: deal with Nix < 2.4 structured attrs

Nix does not (as far it is documented) guarantee that NIX_ATTRS_*_FILE
is set, the only [documented] guarantee seems to be:

> […] made available to the builder via the file .attrs.json in the
> builder’s temporary directory.

This guarantee is of course affected by https://github.com/NixOS/nix/issues/6736,
so it seems to be prudent to fall back to the Nix 2.3 style ATTRS_*_FILE
env vars before defaulting to the expected location in case neither is
available.

See also:

- https://github.com/NixOS/nixpkgs/pull/214937#discussion_r1178101895
- https://github.com/nixos/nixpkgs/commit/afef6588e250

[documented]: https://nixos.org/manual/nix/stable/language/advanced-attributes.html#adv-attr-structuredAttrs

pkgs/stdenv/generic/setup.sh

@@ -29,13 +29,23 @@ if [ -n "$__structuredAttrs" ]; then
         export "$outputName=${outputs[$outputName]}"
     done
 
+    # Before Nix 2.4, $NIX_ATTRS_*_FILE was named differently:
+    # https://github.com/NixOS/nix/commit/27ce722
+    if [[ -n "${ATTRS_JSON_FILE:-}" ]]; then
+        export NIX_ATTRS_JSON_FILE="$ATTRS_JSON_FILE"
+    fi
+
+    if [[ -n "${ATTRS_SH_FILE:-}" ]]; then
+        export NIX_ATTRS_SH_FILE="$ATTRS_SH_FILE"
+    fi
+
     # $NIX_ATTRS_JSON_FILE pointed to the wrong location in sandbox
     # https://github.com/NixOS/nix/issues/6736; please keep around until the
     # fix reaches *every patch version* that's >= lib/minver.nix
-    if ! [[ -e "$NIX_ATTRS_JSON_FILE" ]]; then
+    if ! [[ -e "${NIX_ATTRS_JSON_FILE:-}" ]]; then
         export NIX_ATTRS_JSON_FILE="$NIX_BUILD_TOP/.attrs.json"
     fi
-    if ! [[ -e "$NIX_ATTRS_SH_FILE" ]]; then
+    if ! [[ -e "${NIX_ATTRS_SH_FILE:-}" ]]; then
         export NIX_ATTRS_SH_FILE="$NIX_BUILD_TOP/.attrs.sh"
     fi
 else