set NIX_CFLAGS_COMPILE explicitly to fix CI

linux/nix/build_env.nix

@@ -20,13 +20,20 @@
     ]
     ++ lib.optionals pkgs.stdenv.isLinux [
       wayland
-      # nativeapi includes XInput2.h, which needs these, but xi.pc only lists them privately
-      libxext
-      libxfixes
       (callPackage ./pkg/hook_pkg_config.nix { })
     ];
 
   env = lib.optionalAttrs pkgs.stdenv.isLinux {
+    # nativeapi includes XInput2.h and GTK's Wayland headers, which need these, but xi.pc and gdk-3.0.pc
+    # only list them privately, so its hook's pkg-config doesn't return them. Hooks only get NIX_ variables.
+    NIX_CFLAGS_COMPILE = lib.concatMapStringsSep " " (pkg: "-isystem ${lib.getDev pkg}/include") (
+      with pkgs;
+      [
+        libxext
+        libxfixes
+        wayland
+      ]
+    );
     # better_player loads libmpv at runtime to play videos
     LD_LIBRARY_PATH = "./build/native_assets/linux:${
       lib.makeLibraryPath [