install: Reject a config stateroot on composefs-only images

[andrewdunndev]

The composefs backend only uses the default stateroot. #2544 made --stateroot fail with it and left a stateroot from the install configuration unchecked, since an image may set one for its ostree installs. An image that ships a UKI, or selects composefs itself with no ostree prepare-root.conf, can't be installed with ostree, so its configured stateroot was always ignored and the install still landed in default.

This checks the configured stateroot for those images too, unless --stateroot is passed. With --composefs-backend on an image that can also be installed with ostree, it's still ignored.

flowchart TD
    A["prepare_install loads the install configuration<br/>stateroot = myroot"] --> B{"composefs_required (UKI) or<br/>defaults_to_composefs_backend?"}
    B -- no --> C["config_stateroot is None<br/>kept for ostree installs"]
    B -- yes --> D{"--stateroot passed?"}
    D -- yes --> E["Only the --stateroot check applies"]
    D -- no --> F["InstallComposefsOpts::validate fails<br/>stateroot myroot in the install configuration<br/>is not supported with the composefs backend,<br/>which only uses default"]

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Testing: test_composefs_opts_validate gets install configuration cases. On a Fedora 44 VM, with this change on #2544's branch, bootc install to-disk of an image that selects composefs and sets stateroot = "myroot" exited 1 with the new error and left no partition table, and with --stateroot default it installed into state/os/default. An ostree-capable image with the same configuration installed as before on both backends. An image with a UKI wasn't tested.

Assisted-by: AI

[Johan-Liebert1]

This can be simplified to

Suggested change

	if let (None, Some(stateroot)) = (stateroot, config_stateroot.filter(|&s| s != default))
	if let Some(stateroot) = config_stateroot.filter(|&s| s != default)

since we're already checking stateroot in the above if?