Function compile_app_with_skin 

pub fn compile_app_with_skin(
    dir: &Path,
    parser: &dyn SourceParser,
    plugins: &dyn CompilerPlugins,
    skin: Option<&str>,
    deps: &CompileDeps,
    warnings: &mut Vec<String>,
) -> Result<CompiledApp, RunError>

compile_app with the skin named outright instead of read from lumen.toml.

One caller needs that: a site is built once and served to every OS, so it cannot let [skin] name = "auto" resolve against whichever machine ran the build. Everything else compiles an app for the machine it will run on and calls compile_app.