use create_dir_all instead of create_dir as that doesn't fail if the directory already exists, and make sure to propagate the error out

build.rs

@@ -1,8 +1,8 @@
 use clap_complete::{generate_to, shells::*};
+use clap_mangen::Man;
+use std::fs::File;
 use std::io::Error;
 use std::path::Path;
-use std::fs::File;
-use clap_mangen::Man;
 
 include!("src/cli.rs");
 
@@ -18,8 +18,8 @@ fn main() -> Result<(), Error> {
     generate_to(Elvish, &mut cmd, app_name, outdir)?;
 
     let file = Path::new("man-page").join("dust.1");
-    std::fs::create_dir("man-page");
-    let mut file = File::create(&file)?;
+    std::fs::create_dir_all("man-page")?;
+    let mut file = File::create(file)?;
 
     Man::new(cmd).render(&mut file)?;