git-ignore: split outputs

Use a separate `man` output[1]. All outputs will be installed when using
the package i.e. with `nix-env -f . -iA gitAndTools.git-ignore`.

[1] https://nixos.org/nix/manual/#ssec-derivation

pkgs/applications/version-management/git-and-tools/git-ignore/default.nix

@@ -21,9 +21,10 @@ buildRustPackage rec {
     darwin.apple_sdk.frameworks.Security
   ];
 
+  outputs = [ "out" "man" ];
   preFixup = ''
-    mkdir -p "$out/man/man1"
+    mkdir -p "$man/man/man1"
-    cp target/release/build/git-ignore-*/out/git-ignore.1 "$out/man/man1/"
+    cp target/release/build/git-ignore-*/out/git-ignore.1 "$man/man/man1/"
   '';
 
   meta = with stdenv.lib; {