doc/stdenv: add a note about `__structuredAttrs` (#509970)

  "var-stdenv-enableParallelBuilding": [

    "index.html#var-stdenv-enableParallelBuilding"

  ],

  "var-stdenv-__structuredAttrs": [

    "index.html#var-stdenv-__structuredAttrs"

  ],

  "mkderivation-recursive-attributes": [

    "index.html#mkderivation-recursive-attributes"

  ],

Unless set to `false`, some build systems with good support for parallel building including `cmake`, `meson`, and `qmake` will set it to `true`.

#### `__structuredAttrs` {#var-stdenv-__structuredAttrs}

`__structuredAttrs` defines how derivation attributes are passed to the builder.

If enabled, a shell script and a JSON representation of the derivation attributes are created.

The environment variables {env}`NIX_ATTRS_SH_FILE` and {env}`NIX_ATTRS_JSON_FILE` point to the exact location of these files.

Attributes intended to be _exported_ as environment variables must be defined in the `env` attribute.

Attributes that are _local_ to the buildscript should be defined outside of `env`, to benefit from structured shell variables.

::: {.important}

`__structuredAttrs` is a complete replacement for the way attributes are handled currently, and is the preferred default.

`passAsFile` is disabled when `__structuredAttrs` is enabled, since {env}`NIX_ATTRS_JSON_FILE` can be read from instead.

All new top level packages must enable `__structuredAttrs`.

:::

See the upstream nix documentation for more detail:

  - [Advanced Derivation Attributes](https://nix.dev/manual/nix/2.34/language/advanced-attributes.html#adv-attr-structuredAttrs)

  - [Builder Execution](https://nix.dev/manual/nix/2.34/store/building.html#builder-execution)

  - [Structured Attributes](https://nix.dev/manual/nix/2.34/store/derivation/#structured-attrs)

### Fixed-point arguments of `mkDerivation` {#mkderivation-recursive-attributes}

If you pass a function to `mkDerivation`, it will call the function with an argument that represents the final state of the package: the return value of the function itself, with any overrides applied, as the function is reinvoked by any `overrideAttrs` calls. For example: