handle rustc args properly in bootstrap

Because `RUSTFLAGS` gets overwritten during the conversion from `Cargo` to `Command`,
the passed rustc args were being lost. This change combines the rustc args with the values
that override `RUSTFLAGS`.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/builder.rs

@@ -2094,11 +2094,9 @@ impl<'a> Builder<'a> {
         }
 
         // set rustc args passed from command line
-        let rustc_args =
-            self.config.cmd.rustc_args().iter().map(|s| s.to_string()).collect::<Vec<_>>();
-        if !rustc_args.is_empty() {
-            cargo.env("RUSTFLAGS", &rustc_args.join(" "));
-        }
+        self.config.cmd.rustc_args().iter().for_each(|v| {
+            rustflags.arg(v);
+        });
 
         Cargo {
             command: cargo,