eternal-terminal: move CXXFLAGS into env for structuredAttrs

pkgs/by-name/et/eternal-terminal/package.nix

@@ -45,9 +45,9 @@ stdenv.mkDerivation (finalAttrs: {
     "-DDISABLE_CRASH_LOG=TRUE"
   ];
 
-  CXXFLAGS = lib.optionals stdenv.cc.isClang [
-    "-std=c++17"
-  ];
+  env = lib.optionalAttrs stdenv.cc.isClang {
+    CXXFLAGS = toString [ "-std=c++17" ];
+  };
 
   doCheck = true;