doc/build-helpers: refactor the paragraph about `runCommandLocal`

:   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.

:   If set to `true` this forces the derivation to be built locally, not using [substitutes] nor remote builds.

    This is intended for very cheap commands (<1s execution time) which can be sped up by avoiding the network round-trip(s).

    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

   This prevents the use of [substituters][substituter], 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`.

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

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

[substituter]: https://nix.dev/manual/nix/latest/glossary#gloss-substituter

[substitutes]: https://nix.dev/manual/nix/2.23/glossary#gloss-substitute

::: {.example #ex-runcommandwith}

# Invocation of `runCommandWith`

`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.

`runCommandLocal` is also similar to `runCommand`, but forces the derivation to be built locally.

See the note on [`runCommandWith`] about `runLocal`.

[`runCommandWith`]: #trivial-builder-runCommandWith

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

```

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

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

runCommandLocal :: String -> AttrSet -> String -> Derivation

```

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

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

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

:::

## `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.

::: {.note}

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}

Nixpkgs provides the following functions for producing derivations which write text files or executable scripts into the Nix store.