install: Ensure we label `/` (and `/boot`) #720

Job	Run time
cargo-deny	20s
build-c9s	3m 18s
build-fedora	4m 20s
Build skopeo git main for ubuntu	2m 3s
tests	5m 21s
Test install-alongside	55s
Privileged testing	25s
Container testing	43s
	17m 25s

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