tests.fetchgit.fetchTags: fix flaky test by removing .git directory after getting tags (#413311)

pkgs/build-support/fetchgit/tests.nix

···
     rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
     fetchTags = true;
     leaveDotGit = true;
-    sha256 = "sha256-2vfZnYjZlnC8ODz6B6aOqAqtb1Wbjojnn/5TmzwUrmo=";
+    sha256 = "sha256-y7l+46lVP2pzJwGON5qEV0EoxWofRoWAym5q9VXvpc8=";
     postFetch = ''
       cd $out && git describe --tags --always > describe-output.txt 2>&1 || echo "git describe failed" > describe-output.txt
+      # See https://github.com/NixOS/nixpkgs/issues/412967#issuecomment-2927452118
+      rm -rf .git
     '';
   };
 }