install: Generalize `root-fs-type` into `install.filesystem.root.type` #717

Job	Run time
build-fedora	4m 11s
cargo-deny	24s
Build skopeo git main for ubuntu	1m 47s
build-c9s	3m 36s
tests	5m 2s
Container testing	47s
Test install-alongside	54s
Privileged testing	26s
	17m 7s

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