git: fix darwin build (#103774)

The install step for the darwin specific $out/etc/gitconfig file
assumed that $out/etc already existed. This assumption is no longer
true after f5e2ff1ed80dac74bfb0afa92f34641cce098a49 in #103421. This
change makes make sure the directory exists before adding to it.

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

@@ -249,6 +249,7 @@ stdenv.mkDerivation {
      '')
    + stdenv.lib.optionalString stdenv.isDarwin ''
     # enable git-credential-osxkeychain by default if darwin
+    mkdir -p $out/etc
     cat > $out/etc/gitconfig << EOF
     [credential]
       helper = osxkeychain