checkout: Only verify digest if repo requires fsverity

checkout: Only verify digest if repo requires fsverity

Fixes a regression from the previous commit; in
the case where the target repo doesn't have composefs in
signed mode there's no reason to verify the digest
at checkout time because we aren't verifying it at
boot time either.

The regression is in cases that use rpm-ostree e.g.
where as of recently we unconditionally add the composefs
digest, but for e.g. FCOS we aren't deploying with fsverity
enabled.

Closes: https://github.com/ostreedev/ostree/issues/3330
Signed-off-by: Colin Walters <walters@verbum.org>