doc/build-helpers: refactor the paragraphs about `runCommand{,CC}`

:::

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

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

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

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

The result of `runCommand name drvAttrs buildCommand` is a derivation that is built by running the specified shell commands.

`runCommandCC` is similar but uses the default compiler environment. To minimize dependencies, `runCommandCC`

should only be used when the build command needs a C compiler.

By default `runCommand` runs in a stdenv with no compiler environment, whereas [`runCommandCC`](#trivial-builder-runCommandCC) uses the default stdenv, `pkgs.stdenv`.

### Type {#trivial-builder-runCommand-Type}

`name :: String`

:   The name that Nix will append to the store path in the same way that `stdenv.mkDerivation` uses its `name` attribute.

```

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

runCommandCC :: String -> AttrSet -> String -> Derivation

```

`drvAttr :: AttrSet`

:   Attributes to pass to the underlying call to [`stdenv.mkDerivation`](#chap-stdenv).

### Input {#trivial-builder-runCommand-Input}

`buildCommand :: String`

:   Shell commands to run in the derivation builder.

While the type signature(s) differ from `runCommandWith`, individual arguments with the same name will have the same type and meaning:

`name` (String)

:   The derivation's name

`derivationArgs` (Attribute set)

:   Additional parameters passed to [`mkDerivation`]

`buildCommand` (String)

:   The command(s) run to build the derivation.

    ::: {.note}

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

    :::

::: {.example #ex-runcommand-simple}

# Invocation of `runCommand`

```nix

(import <nixpkgs> {}).runCommand "my-example" {} ''

runCommand "my-example" {} ''

  echo My example command is running

  mkdir $out

```

:::

## `runCommandCC` {#trivial-builder-runCommandCC}

This works just like `runCommand`. The only difference is that it also provides a C compiler in `buildCommand`'s environment. To minimize your dependencies, you should only use this if you are sure you will need a C compiler as part of running your command.

## `runCommandLocal` {#trivial-builder-runCommandLocal}

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.