Building: Normalizing Derivation Inputs and Builder Execution

# Building ## Normalizing derivation inputs - Each input must be [realised] prior to building the derivation in question. - Once this is done, the derivation is *normalized*, replacing each input deriving path with its store path, which we now know from realising the input. ## Builder Execution The [`builder`](./derivation/index.md#builder) is executed as follows: - A temporary directory is created under the directory specified by `TMPDIR` (default `/tmp`) where the build will take place. The current directory is changed to this directory. - The environment is cleared and set to the derivation attributes, as specified above. - In addition, the following variables are set: - `NIX_BUILD_TOP` contains the path of the temporary directory for this build. - Also, `TMPDIR`, `TEMPDIR`, `TMP`, `TEMP` are set to point to the temporary directory. This is to prevent the builder from accidentally writing temporary files anywhere else. Doing so might cause interference by other processes. - `PATH` is set to `/path-not-set` to prevent shells from initialising it to their built-in default value. - `HOME` is set to `/homeless-shelter` to prevent programs from using `/etc/passwd` or the like to find the user's home directory, which could cause impurity. Usually, when `HOME` is set, it is used as the location of the home directory, even if it points to a non-existent path. - `NIX_STORE` is set to the path of the top-level Nix store directory (typically, `/nix/store`). - `NIX_ATTRS_JSON_FILE` & `NIX_ATTRS_SH_FILE` if `__structuredAttrs` is set to `true` for the derivation. A detailed explanation of this behavior can be found in the [section about structured attrs](@docroot@/language/advanced-attributes.md#adv-attr-structuredAttrs). - For each output declared in `outputs`, the corresponding environment variable is set to point to the intended path in the Nix store for that output. Each output path is a concatenation

Before building a derivation, its inputs are realized and normalized by replacing derivation paths with store paths. The builder is then executed in a temporary directory with a cleared environment set to derivation attributes, along with specific variables like NIX_BUILD_TOP, TMPDIR, PATH, HOME, and NIX_STORE to ensure a clean and controlled build process. Structured attributes also influence the environment through `NIX_ATTRS_JSON_FILE` & `NIX_ATTRS_SH_FILE`.