You don't need y.sh prepare to build, only to test

Also improve the error message when trying to test without having run
y.sh prepare first.

Readme.md

@@ -49,13 +49,13 @@ If you want to build the backend manually, you can download it from GitHub and b
 ```bash
 $ git clone https://github.com/rust-lang/rustc_codegen_cranelift
 $ cd rustc_codegen_cranelift
-$ ./y.sh prepare
 $ ./y.sh build
 ```
 
 To run the test suite replace the last command with:
 
 ```bash
+$ ./y.sh prepare # only needs to be run the first time
 $ ./test.sh
 ```
 

build_system/prepare.rs

@@ -91,6 +91,13 @@ impl GitRepo {
 
     fn verify_checksum(&self, dirs: &Dirs) {
         let download_dir = self.download_dir(dirs);
+        if !download_dir.exists() {
+            eprintln!(
+                "Missing directory {download_dir}: Please run ./y.sh prepare to download.",
+                download_dir = download_dir.display(),
+            );
+            std::process::exit(1);
+        }
         let actual_hash = format!("{:016x}", hash_dir(&download_dir));
         if actual_hash != self.content_hash {
             eprintln!(