Fix and test launching without --runtime

• pressure-vessel: Fix sense of assertion, and realign with other checks

  This function won't work in Flatpak, so we make sure that we only call it when we are not in Flatpak. The assertion should match that.

  This fixes the ability to run:

    ./pressure-vessel/bin/pressure-vessel-wrap -- xterm

  when not in a Flatpak environment, which in principle is meant to work (at least on reasonably normal FHS operating systems), although it isn't regularly tested.

  Thanks to NixOS contributor moben on Github, who spotted this while debugging.

• pressure-vessel: Add minimal test coverage for absence of --runtime

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/cc @denittis

Split out from !176 (merged).