Bump hackage-overlay-repo-tool

This clamps the mtime of files in patched source tarballs to the mtime
of the patch to ensure that the time of the build doesn't sneak into the
hashes in the index.

ci/default.nix

@@ -17,8 +17,8 @@ let
       src = fetchFromGitHub {
         owner = "bgamari";
         repo = "hackage-overlay-repo-tool";
-        rev = "18eb61c830ad908d36d343f400a1588af6b9a03a";
-        sha256 = "1y1fw5x9lyd533lm67s7iyzb4640y8lya11sdjia0yd1j5if6s40";
+        rev = "7aac81e9bc468b103dd78b9c662672c86fe236f7";
+        sha256 = "0i4iw8nbhvc2xx05c0hbnnjyhap3b4xsclmxnmfa6dsa2ym02jc0";
       };
     in haskellPackages.callCabal2nix "hackage-overlay-repo-tool" src {};