installer: don't assume GNU diff

macOS Ventura ships with it's own version of diff. Try to output a
similar diff with Apple diff as with GNU diff, instead of failing

Helps NixOS#7286

scripts/install-multi-user.sh

@@ -246,8 +246,15 @@ printf -v _OLD_LINE_FMT "%b" $'\033[1;7;31m-'"$ESC ${RED}%L${ESC}"
 printf -v _NEW_LINE_FMT "%b" $'\033[1;7;32m+'"$ESC ${GREEN}%L${ESC}"
 
 _diff() {
+    # macOS Ventura doesn't ship with GNU diff. Print similar output except
+    # without +/- markers or dimming
+    if diff --version | grep -q "Apple diff"; then
+        printf -v CHANGED_GROUP_FORMAT "%b" "${GREEN}%>${RED}%<${ESC}"
+        diff --changed-group-format="$CHANGED_GROUP_FORMAT" "$@"
+    else
     # simple colorized diff comatible w/ pre `--color` versions
-    diff --unchanged-group-format="$_UNCHANGED_GRP_FMT" --old-line-format="$_OLD_LINE_FMT" --new-line-format="$_NEW_LINE_FMT" --unchanged-line-format="  %L" "$@"
+        diff --unchanged-group-format="$_UNCHANGED_GRP_FMT" --old-line-format="$_OLD_LINE_FMT" --new-line-format="$_NEW_LINE_FMT" --unchanged-line-format="  %L" "$@"
+    fi
 }
 
 confirm_rm() {