exports: Never try to export paths below /run/flatpak or /run/host

These directories are reserved for Flatpak's own use.

Helps: https://github.com/flatpak/flatpak/issues/5205
Signed-off-by: Simon McVittie <smcv@collabora.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f