make qml cache disabling conditional on statvfs

Merged Harald Sitter requested to merge work/sitter/conditionalcache into master

this is still a bit risky but 5GiB hopefully gives us enough leeway to create valid caches before potentially running out of space. e.g. when the system is currently leaking onto a disk somehow, the space could not survive for very long. the hope is that this is enough of an edge case that it should not ever happen in practice.

filesystem::space should be supported on all supported platforms

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