fix: Make work on Windows

This works around rust-lang/rustup#3036 but also makes it so it can work
without rustup.

src/main.rs

@@ -1077,17 +1077,29 @@ fn cargo(
     script_args: &[OsString],
     run_quietly: bool,
 ) -> MainResult<Command> {
-    let mut cmd = Command::new("cargo");
+    let mut cmd = if std::env::var_os("RUSTUP_TOOLCHAIN").is_some() {
+        let mut cmd = Command::new("rustup");
+        // Always specify a toolchain to avoid being affected by rust-version(.toml) files:
+        let toolchain_version = toolchain_version.unwrap_or("stable");
+        cmd.args(["run", toolchain_version, "cargo"]);
+        cmd
+    } else {
+        if let Some(toolchain_version) = toolchain_version {
+            return Err(format!(
+                "--toolchain-version={} cannot be used without rustup being available",
+                toolchain_version
+            )
+            .into());
+        }
+        Command::new("cargo")
+    };
 
     // Set tracing on if not set
     if std::env::var_os("RUST_BACKTRACE").is_none() {
         cmd.env("RUST_BACKTRACE", "1");
         info!("setting RUST_BACKTRACE=1 for this cargo run");
     }
 
-    // Always specify a toolchain to avoid being affected by rust-version(.toml) files:
-    cmd.arg(format!("+{}", toolchain_version.unwrap_or("stable")));
-
     cmd.arg(cmd_name);
 
     if cmd_name == "run" && run_quietly {