flutter: drop moreutils for more bash

Since moreutils depends on Perl and that causes #315990, drop moreutils
until Perl is fixed to handle cross compiling properly. We now create a
temporary file and write the jq result to it and then copy that to
`$out`.

pkgs/build-support/flutter/default.nix

@@ -10,7 +10,6 @@
 , pkg-config
 , jq
 , yq
-, moreutils
 }:
 
 # absolutely no mac support for now
@@ -73,11 +72,15 @@ let
       extraPackageConfigSetup = ''
         # https://github.com/flutter/flutter/blob/3.13.8/packages/flutter_tools/lib/src/dart/pub.dart#L755
         if [ "$('${yq}/bin/yq' '.flutter.generate // false' pubspec.yaml)" = "true" ]; then
+          export TEMP_PACKAGES=$(mktemp)
           '${jq}/bin/jq' '.packages |= . + [{
             name: "flutter_gen",
             rootUri: "flutter_gen",
             languageVersion: "2.12",
-          }]' "$out" | '${moreutils}/bin/sponge' "$out"
+          }]' "$out" > "$TEMP_PACKAGES"
+          cp "$TEMP_PACKAGES" "$out"
+          rm "$TEMP_PACKAGES"
+          unset TEMP_PACKAGES
         fi
       '';
     };