Nix Builtin Function Extensions and Build Environment Updates

differs from the source path name (e.g. `builtins.path { path = ./foo; name = "bar"; }`) and also supports filtering out unwanted files. - `builtins.fetchurl` and `builtins.fetchTarball` now support `sha256` and `name` attributes. - `builtins.split` splits a string using a POSIX extended regular expression as the separator. - `builtins.partition` partitions the elements of a list into two lists, depending on a Boolean predicate. - `<nix/fetchurl.nix>` now uses the content-addressable tarball cache at <http://tarballs.nixos.org/>, just like `fetchurl` in Nixpkgs. (f2682e6e18a76ecbfb8a12c17e3a0ca15c084197) - In restricted and pure evaluation mode, builtin functions that download from the network (such as `fetchGit`) are permitted to fetch underneath a list of URI prefixes specified in the option `allowed-uris`. The Nix build environment has the following changes: - Values such as Booleans, integers, (nested) lists and attribute sets can [now](https://github.com/NixOS/nix/commit/6de33a9c675b187437a2e1abbcb290981a89ecb1) be passed to builders in a non-lossy way. If the special attribute `__structuredAttrs` is set to `true`, the other derivation attributes are serialised in JSON format and made available to the builder via the file `.attrs.json` in the builder’s temporary directory. This obviates the need for `passAsFile` since JSON files have no size restrictions, unlike process environments. [As a convenience to Bash builders](https://github.com/NixOS/nix/commit/2d5b1b24bf70a498e4c0b378704cfdb6471cc699), Nix writes a script named `.attrs.sh` to the builder’s directory that initialises shell variables corresponding to all attributes that are representable in Bash. This includes non-nested (associative) arrays. For example, the attribute `hardening.format = true` ends up as the Bash associative array element `${hardening[format]}`. - Builders can [now](https://github.com/NixOS/nix/commit/88e6bb76de5564b3217be9688677d1c89101b2a3) communicate what build phase they are in by writing messages to the file descriptor specified in `NIX_LOG_FD`. The current phase is shown by the `nix` progress indicator. - In Linux sandbox builds, we [now](https://github.com/NixOS/nix/commit/a2d92bb20e82a0957067ede60e91fab256948b41) provide a default `/bin/sh` (namely `ash` from BusyBox). - In structured attribute mode, `exportReferencesGraph` [exports](https://github.com/NixOS/nix/commit/c2b0d8749f7e77afc1c4b3e8dd36b7ee9720af4a) extended information about closures in JSON format. In particular,

Nix updates include: **Builtin Functions:** `builtins.path` for flexible path handling/filtering; `fetchurl`/`fetchTarball` now support `sha256`/`name`; new `split` (regex string splitting) and `partition` (list segregation). Network fetching builtins can use `allowed-uris` in restricted modes, and a content-addressable tarball cache is utilized. **Build Environment:** Structured attribute passing now serializes complex values (Booleans, integers, lists, sets) as JSON when `__structuredAttrs` is true, bypassing environment size limits. A `.attrs.sh` script assists Bash builders. Builders can communicate build phases via `NIX_LOG_FD`. Linux sandbox builds provide a default `/bin/sh` (BusyBox `ash`). `exportReferencesGraph` now exports extended JSON closure info with structured attributes.