Drop some moves that would happen anyway but forbid NRVO where appicable

src/libexpr/eval.cc

@@ -2290,7 +2290,7 @@ BackedStringView EvalState::coerceToString(
                     && (!v2->isList() || v2->listSize() != 0))
                     result += " ";
             }
-            return std::move(result);
+            return result;
         }
     }
 

src/libexpr/paths.cc

@@ -4,7 +4,7 @@ namespace nix {
 
 SourcePath EvalState::rootPath(CanonPath path)
 {
-    return std::move(path);
+    return path;
 }
 
 }