make codegen-backends directory name configurable

This allows to parallel-install several versions of rust system-wide
Fixes #48263

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

src/bootstrap/builder.rs

@@ -462,7 +462,7 @@ impl<'a> Builder<'a> {
 
     pub fn sysroot_codegen_backends(&self, compiler: Compiler) -> PathBuf {
         self.sysroot_libdir(compiler, compiler.host)
-            .with_file_name("codegen-backends")
+            .with_file_name(self.build.config.rust_codegen_backends_dir.clone())
     }
 
     /// Returns the compiler's libdir where it stores the dynamic libraries that