fix: Use rustup toolchain invocations instead of rustup proxies

On Windows, nested Cargo invocations fail if a toolchain is specified, since the nested invocations do not use the rustup proxy anymore, but use the exact executable instead, see rust-lang/rustup#3036.

To solve this issue, we now build `rustup run TOOLCHAIN cargo` commands, instead of `cargo +TOOLCHAIN`, skipping the proxies entirely.

cargo-mutest/src/main.rs

@@ -68,8 +68,10 @@ fn main() {
         _ => unreachable!(),
     };
 
-    let mut cmd = Command::new("cargo");
-    cmd.arg(format!("+{}", build::RUST_TOOLCHAIN_VERSION));
+    let mut cmd = Command::new("rustup");
+    cmd.arg("run");
+    cmd.arg(build::RUST_TOOLCHAIN_VERSION);
+    cmd.arg("cargo");
     cmd.arg(cargo_subcommand);
     cmd.args(cargo_args);