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

Changed files
+10
nixos
modules
+10
nixos/modules/installer/sd-card/sd-image-riscv64-qemu-installer.nix
···
···
+
{
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imports = [
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../../profiles/installation-device.nix
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./sd-image-riscv64-qemu.nix
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];
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# the installation media is also the installation target,
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# so we don't want to provide the installation configuration.nix.
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installer.cloneConfig = false;
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}