checks: show Sequoia migration commands for other installers

This is hopefully largely cosmetic, but should increase user
confidence in the safety and effectiveness of the migration, and the
Determinate Systems and Lix installer commands will also update the
`/nix/receipt.json` files to match the changes made. They cannot
properly handle the format of each other’s receipts, so we need to
detect which was used.

modules/system/checks.nix

@@ -72,16 +72,24 @@ let
         printf >&2 'compatibility with macOS Sequoia 15. Your _nixbld1 user currently has\n'
         printf >&2 'UID %d rather than the new default of 351.\n' "$firstBuildUserID"
         printf >&2 '\n'
-        printf >&2 'You can automatically migrate your users using the following script\n'
-        printf >&2 'from the Nix repository:\n'
+        printf >&2 'You can automatically migrate the users with the following command:\n'
         printf >&2 '\n'
-        printf >&2 '    https://github.com/NixOS/nix/raw/master/scripts/sequoia-nixbld-user-migration.sh\n'
-        printf >&2 '\n'
-        printf >&2 'This should work even if you installed Nix with the Determinate\n'
-        printf >&2 'Systems installer or are using Lix. If you are comfortable using the\n'
-        printf >&2 'script without review, you can run:\n'
-        printf >&2 '\n'
-        printf >&2 "    curl --proto '=https' --tlsv1.2 -sSf -L https://github.com/NixOS/nix/raw/master/scripts/sequoia-nixbld-user-migration.sh | bash -\n"
+        if [[ -e /nix/receipt.json ]]; then
+            if
+                ${pkgs.jq}/bin/jq --exit-status \
+                'try(.planner.settings | has("enable_flakes"))' \
+                /nix/receipt.json \
+                >/dev/null
+            then
+                installerUrl="https://install.lix.systems/lix"
+            else
+                installerUrl="https://install.determinate.systems/nix"
+            fi
+            printf >&2 "    curl --proto '=https' --tlsv1.2 -sSf -L %s | sh -s -- repair sequoia --move-existing-users\n" \
+                "$installerUrl"
+        else
+            printf >&2 "    curl --proto '=https' --tlsv1.2 -sSf -L https://github.com/NixOS/nix/raw/master/scripts/sequoia-nixbld-user-migration.sh | bash -\n"
+        fi
         printf >&2 '\n'
         printf >&2 'If you have no intention of upgrading to macOS Sequoia 15, or already\n'
         printf >&2 'have a custom UID range that you know is compatible with Sequoia, you\n'