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.

Author: bjorn3

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
 ```