doc/build-helpers: document `runCommandWith`

Nixpkgs provides a variety of wrapper functions that help build commonly useful derivations.

Like [`stdenv.mkDerivation`](#sec-using-stdenv), each of these build helpers creates a derivation, but the arguments passed are different (usually simpler) from those required by `stdenv.mkDerivation`.

## `runCommandWith` {#trivial-builder-runCommandWith}

The function `runCommandWith` returns a derivation built using the specified command(s), in a specified environment.

It is the underlying base function of  all `runCommand*` variants. 

The general behavior is controlled via a single attribute set passed 

as the first argument, and allows specifying `stdenv` freely.

### Type {#trivial-builder-runCommandWith-Type}

```

runCommandWith :: {

  name :: name;

  stdenv? :: Derivation;

  runLocal? :: Bool;

  derivationArgs? :: { ... };

} -> String -> Derivation

```

### Inputs {#trivial-builder-runCommandWith-Inputs}

`name` (String)

:   The derivation's name, which Nix will append to the store path; see [`mkDerivation`](#sec-using-stdenv).

`runLocal` (Boolean)

:   If set to `true` this forces the derivation to be built locally. Remote substitutes and distributed builds, won't be used.

    Its effect is to set [`preferLocalBuild = true`][preferLocalBuild] and [`allowSubstitutes = false`][allowSubstitutes].

   ::: {.note}

   This prevents the use of substitutors, so only set `runLocal` (or use `runCommandLocal`) when certain the user will

   always have a builder for the `system` of the derivation. This should be true for most trivial use cases

   (e.g., just copying some files to a different location or adding symlinks) because there the `system`

   is usually the same as `builtins.currentSystem`.

   :::

`stdenv` (Derivation)

:   The [standard environment](#chap-stdenv) to use, defaulting to `pkgs.stdenv`

`derivationArgs` (Attribute set)

:   Additional arguments for [`mkDerivation`](#sec-using-stdenv).

`buildCommand` (String)

:   Shell commands to run in the derivation builder.

    ::: {.note}

    You have to create a file or directory `$out` for Nix to be able to run the builder successfully.

    :::

[allowSubstitutes]: https://nixos.org/nix/manual/#adv-attr-allowSubstitutes

[preferLocalBuild]: https://nixos.org/nix/manual/#adv-attr-preferLocalBuild

::: {.example #ex-runcommandwith}

# Invocation of `runCommandWith`

```nix

runCommandWith {

  name = "example";

  derivationArgs.nativeBuildInputs = [ cowsay ];

} ''

  cowsay > $out <<EOMOO

  'runCommandWith' is a bit cumbersome,

  so we have more ergonomic wrappers.

  EOMOO

''

```

:::

## `runCommand` {#trivial-builder-runCommand}

`runCommand :: String -> AttrSet -> String -> Derivation`

Variant of `runCommand` that forces the derivation to be built locally, it is not substituted. This is intended for very cheap commands (<1s execution time). It saves on the network round-trip and can speed up a build.

::: {.note}

This sets [`allowSubstitutes` to `false`](https://nixos.org/nix/manual/#adv-attr-allowSubstitutes), so only use `runCommandLocal` if you are certain the user will always have a builder for the `system` of the derivation. This should be true for most trivial use cases (e.g., just copying some files to a different location or adding symlinks) because there the `system` is usually the same as `builtins.currentSystem`.

This sets [`allowSubstitutes` to `false`][allowSubstitutes], so only use `runCommandLocal` if you are certain the user will always have a builder for the `system` of the derivation. This should be true for most trivial use cases (e.g., just copying some files to a different location or adding symlinks) because there the `system` is usually the same as `builtins.currentSystem`.

:::

## Writing text files {#trivial-builder-text-writing}

  # `runCommandCCLocal` left out on purpose.

  # We shouldn’t force the user to have a cc in scope.

  # TODO: Move documentation for runCommandWith to the Nixpkgs manual

  /*

    Generalized version of the `runCommand`-variants

    which does customized behavior via a single

    attribute set passed as the first argument

    instead of having a lot of variants like

    `runCommand*`. Additionally it allows changing

    the used `stdenv` freely and has a more explicit

    approach to changing the arguments passed to

    `stdenv.mkDerivation`.

   */

  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md

  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-runCommandWith

  runCommandWith =

    let

      # prevent infinite recursion for the default stdenv value