Use beta cargo in opt-dist

Using the new cargo caused issues when a backwards-incompatible change was made to cargo.

src/tools/opt-dist/src/tests.rs

@@ -28,7 +28,6 @@ pub fn run_tests(env: &Environment) -> anyhow::Result<()> {
     let rustc_dir = extract_dist_dir(&format!("rustc-{version}-{host_triple}"))?.join("rustc");
     let libstd_dir = extract_dist_dir(&format!("rust-std-{version}-{host_triple}"))?
         .join(format!("rust-std-{host_triple}"));
-    let cargo_dir = extract_dist_dir(&format!("cargo-{version}-{host_triple}"))?.join("cargo");
     let extracted_src_dir = extract_dist_dir(&format!("rust-src-{version}"))?.join("rust-src");
 
     // We need to manually copy libstd to the extracted rustc sysroot
@@ -47,8 +46,6 @@ pub fn run_tests(env: &Environment) -> anyhow::Result<()> {
 
     let rustc_path = rustc_dir.join("bin").join(format!("rustc{}", executable_extension()));
     assert!(rustc_path.is_file());
-    let cargo_path = cargo_dir.join("bin").join(format!("cargo{}", executable_extension()));
-    assert!(cargo_path.is_file());
 
     // Specify path to a LLVM config so that LLVM is not rebuilt.
     // It doesn't really matter which LLVM config we choose, because no sysroot will be compiled.
@@ -65,13 +62,11 @@ change-id = 115898
 
 [build]
 rustc = "{rustc}"
-cargo = "{cargo}"
 
 [target.{host_triple}]
 llvm-config = "{llvm_config}"
 "#,
         rustc = rustc_path.to_string().replace('\\', "/"),
-        cargo = cargo_path.to_string().replace('\\', "/"),
         llvm_config = llvm_config.to_string().replace('\\', "/")
     );
     log::info!("Using following `config.toml` for running tests:\n{config_content}");