Rollup merge of #115991 - ferrocene:rustdoc-themes, r=pietroalbini

Ensure `build/tmp` exists in `rustdoc_themes::get_themes`

This causes failures in ferrocene's CI as `build/tmp` might not exist at this point, causing the following expect to fail here https://github.com/rust-lang/rust/blob/4b91288484569dd59c9a996ae45e08ae9527abc1/src/tools/rustdoc-themes/main.rs#L24

Author: GuillaumeGomez