Make pressure-vessel-wrap responsible for unpacking runtimes

This simplifies the startup scripts a lot: deploy-runtime and run-in-steamrt no longer need to exist (although I've left behind stub versions to make it easier to switch between versions during testing), and we are not trying to do locking from shell script code.

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I think this is ready for review, but it requires steam-runtime-tools!239 (merged) so cannot be merged until we have a release with that.