sd-image-riscv64: Add an -installer variant like others

nixos/modules/installer/sd-card/sd-image-riscv64-qemu-installer.nix

···
+{
+  imports = [
+    ../../profiles/installation-device.nix
+    ./sd-image-riscv64-qemu.nix
+  ];
+
+  # the installation media is also the installation target,
+  # so we don't want to provide the installation configuration.nix.
+  installer.cloneConfig = false;
+}