STORAGE_BACKEND setting: use "local" if unspecified

the implementation now matches what is the README specifies

src/storage.rs

@@ -27,19 +27,19 @@ impl Storage {
     }
 
     pub async fn from_env() -> anyhow::Result<Self> {
-        match env_var("STORAGE_BACKEND")?.as_ref() {
-            "local" => {
+        match env_var("STORAGE_BACKEND").as_deref() {
+            Ok("local") | Err(_) => {
                 let dir = match env_var("STORAGE_LOCAL_DIR") {
                     Ok(dir) => dir,
                     Err(_) => ".".to_string(),
                 };
                 Ok(Self::from_local_dir(PathBuf::from(dir)))
             }
-            "s3" => {
+            Ok("s3") => {
                 let bucket = env_var("STORAGE_S3_BUCKET")?;
                 Ok(Self::from_s3(bucket).await)
             }
-            other => {
+            Ok(other) => {
                 anyhow::bail!("STORAGE_BACKEND: unexpected value {other} (expected: local/s3)")
             }
         }