treewide: NIX_*_COMPILE -> string

pkgs/development/compilers/rust/rustc.nix

@@ -41,11 +41,11 @@ in stdenv.mkDerivation rec {
   # See: https://github.com/NixOS/nixpkgs/pull/56540#issuecomment-471624656
   stripDebugList = [ "bin" ];
 
-  NIX_LDFLAGS =
+  NIX_LDFLAGS = toString (
        # when linking stage1 libstd: cc: undefined reference to `__cxa_begin_catch'
        optional (stdenv.isLinux && !withBundledLLVM) "--push-state --as-needed -lstdc++ --pop-state"
     ++ optional (stdenv.isDarwin && !withBundledLLVM) "-lc++"
-    ++ optional stdenv.isDarwin "-rpath ${llvmSharedForHost}/lib";
+    ++ optional stdenv.isDarwin "-rpath ${llvmSharedForHost}/lib");
 
   # Increase codegen units to introduce parallelism within the compiler.
   RUSTFLAGS = "-Ccodegen-units=10";