Previously, --filesystem=/run would prevent apps from starting by
breaking our ability to set up /run/flatpak and /run/host. Now it is
ignored, with a diagnostic message, resolving #5205 and #5207.
Similarly, --filesystem=/symlink-to-root (or --filesystem=host) would
have prevented apps from starting if a symlink like
`/symlink-to-root -> /` or `/symlink-to-root -> .` exists, and refusing
to export the target of that symlink avoids that failure mode,
resolving #1357.
Resolves: https://github.com/flatpak/flatpak/issues/1357
Resolves: https://github.com/flatpak/flatpak/issues/5205
Resolves: https://github.com/flatpak/flatpak/issues/5207
Signed-off-by: Simon McVittie <smcv@collabora.com>
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