feat(builder): tighten sandbox + reap orphan transient units #18

Job	Run time
prep	4s
build (ubuntu-24.04, x86_64-unknown-linux-gnu, amd64)	2m 22s
build (ubuntu-24.04-arm, aarch64-unknown-linux-gnu, arm64)	1m 43s
release	12s
	4m 21s

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