Avoid re-fetching Unicode data

If the unicode-downloads folder already exists, we likely just fetched the data,
so don't make any further network requests. Unicode versions are released rarely
enough that this doesn't matter much in practice.

src/tools/unicode-table-generator/src/unicode_download.rs

@@ -11,10 +11,15 @@ static RESOURCES: &[&str] =
 
 pub fn fetch_latest() {
     let directory = Path::new(UNICODE_DIRECTORY);
+    if directory.exists() {
+        eprintln!(
+            "Not refetching unicode data, already exists, please delete {:?} to regenerate",
+            directory
+        );
+        return;
+    }
     if let Err(e) = std::fs::create_dir_all(directory) {
-        if e.kind() != std::io::ErrorKind::AlreadyExists {
-            panic!("Failed to create {:?}: {}", UNICODE_DIRECTORY, e);
-        }
+        panic!("Failed to create {:?}: {}", UNICODE_DIRECTORY, e);
     }
     let output = Command::new("curl").arg(URL_PREFIX.to_owned() + README).output().unwrap();
     if !output.status.success() {