fetchgit: require sparseCheckout be a list of strings

Passing a (multi-line) string was deprecated in #200082 in favour of
list of strings, but still supported (with warning). Now, enforce use of
list of strings.

pkgs/build-support/fetchgithub/default.nix

@@ -25,7 +25,7 @@ let
   };
   passthruAttrs = removeAttrs args [ "owner" "repo" "rev" "fetchSubmodules" "forceFetchGit" "private" "githubBase" "varPrefix" ];
   varBase = "NIX${if varPrefix == null then "" else "_${varPrefix}"}_GITHUB_PRIVATE_";
-  useFetchGit = fetchSubmodules || (leaveDotGit == true) || deepClone || forceFetchGit || !(sparseCheckout == "" || sparseCheckout == []);
+  useFetchGit = fetchSubmodules || (leaveDotGit == true) || deepClone || forceFetchGit || (sparseCheckout != []);
   # We prefer fetchzip in cases we don't need submodules as the hash
   # is more stable in that case.
   fetcher = if useFetchGit then fetchgit else fetchzip;