fix(script): Split cargo script's target-dir from build-dir by default

This avoids needing to be careful about cargo scripts conflicting with
"artifact names".
Granted, there will be more work to deal with the validation checks but
that comes later.

Author: epage

src/cargo/core/workspace.rs

@@ -447,7 +447,7 @@ impl<'gctx> Workspace<'gctx> {
 
     fn default_target_dir(&self) -> Filesystem {
         if self.root_maybe().is_embedded() {
-            self.default_build_dir()
+            self.build_dir().join("target")
         } else {
             Filesystem::new(self.root().join("target"))
         }