patching: log warning when rust2rpm-helper is not in PATH

rust2rpm/patching.py

@@ -70,6 +70,10 @@ def preprocess_cargo_toml(contents: str, features: set[str]) -> Optional[str]:
     if shutil.which("rust2rpm-helper"):
         return preprocess_cargo_toml_helper(contents)
     else:
+        log.warn(
+            "Falling back to broken built-in implementation for stripping non-applicable "
+            + "target-specific depdendencies: rust2rpm-helper is not installed"
+        )
         return preprocess_cargo_toml_fallback(contents, features)