feat: add harmonia cache server

features/harmonia/cache-builder.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -uo pipefail
+
+build() {
+	local name="$1" ref="$2" link="$3"
+	echo "building $name..." >&2
+	if nix build "$ref" --out-link "$link"; then
+		return 0
+	else
+		echo "FAILED: $name" >&2
+		return 1
+	fi
+}
+
+main() {
+	mkdir -p "$GC_ROOT_DIR"
+	local failed=0
+
+	for host in $HOSTS; do
+		build "$host" \
+			"$FLAKE_REF#nixosConfigurations.$host.config.system.build.toplevel" \
+			"$GC_ROOT_DIR/$host" || failed=1
+	done
+
+	build "ephvm-image" \
+		"$FLAKE_REF#nixosConfigurations.ephvm.config.system.build.images.qemu" \
+		"$GC_ROOT_DIR/ephvm-image" || failed=1
+
+	return $failed
+}
+
+main "$@"