install: Ensure we label `/` (and `/boot`) #721

Job	Run time
cargo-deny	21s
Build skopeo git main for ubuntu	2m 5s
build-fedora	4m 19s
build-c9s	3m 22s
tests	5m 10s
Privileged testing	25s
Test install-alongside	53s
Container testing	42s
	17m 17s

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