propagate --target only if target is not host

Passing `--target` alters `RUSTFLAGS` in the build scripts. Since there is no reason
to pass them when building for the host triple, exclude them if the target is the host triple.

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

src/bootstrap/src/core/builder.rs

@@ -1343,10 +1343,9 @@ impl<'a> Builder<'a> {
             Color::Auto => {} // nothing to do
         }
 
-        if cmd != "install" {
+        if target != compiler.host {
+            assert!(cmd != "install", "`x install` can only be called for the host triple.");
             cargo.arg("--target").arg(target.rustc_target_arg());
-        } else {
-            assert_eq!(target, compiler.host);
         }
 
         if self.config.rust_optimize.is_release() {