Fix cargo vendor not working without rustc installed

This fixes an issue where vendoring would fail if `rustc` is not in
PATH. This happens with the update of some workspaces to the 2024
edition which need to run `rustc -Vv` to determine which rust version is
in use.

Author: ehuss

src/bootstrap/src/core/build_steps/vendor.rs

@@ -103,6 +103,7 @@ impl Step for Vendor {
         // Will read the libstd Cargo.toml
         // which uses the unstable `public-dependency` feature.
         cmd.env("RUSTC_BOOTSTRAP", "1");
+        cmd.env("RUSTC", &builder.initial_rustc);
 
         cmd.current_dir(self.root_dir).arg(&self.output_dir);