nix: set nim cache to proper tmp directory

Author: siddarthkay

nix/shell.nix

@@ -23,5 +23,6 @@ in pkgs.mkShell {
   # Avoid compiling Nim itself.
   shellHook = ''
     export USE_SYSTEM_NIM=1
+    export XDG_CACHE_HOME="$TMPDIR"
   '';
 }