Fix typo in NIX_AGDA_STDLIB environment variable (#7352)

Author: basetunnel

nix/agda-tools.nix

@@ -142,7 +142,7 @@ let
   };
 
   # Path to the stdlib .agda-lib file for shell export.
-  NIX_AGDA_STDLIB = "${agda-stdlib}/stadard-library.agda-lib";
+  NIX_AGDA_STDLIB = "${agda-stdlib}/standard-library.agda-lib";
 
 in