fix(userspace/engine): support appending to unknown sources #1818

Job	Run time
fetch-version / fetch-version	25s
build-dev / build-and-test	9m 47s
build-dev-minimal / build-and-test	4m 15s
build-dev-packages / build-wasm-package	8m 14s
build-dev-packages / build-musl-package	50m 56s
build-dev-packages / build-modern-bpf-skeleton	2m 40s
check-engine-version	4s
check-engine-checksum	5s
build-dev-packages / build-packages	49m 39s
test-dev-packages (static) / test-packages	2m 0s
test-dev-packages / test-packages	2m 4s
	2h 10m 9s

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