portal: Remap --env-fd into child process's fd space
portal: Remap --env-fd into child process's fd space

Just because we can allocate a new, unused fd in the portal's fd space,
that doesn't mean that fd number is going to be unused in the child
process's fd space: we might need to remap it.

Resolves: flatpak/flatpak#4286
Fixes: aeb6a7ab "portal: Convert --env in extra-args into --env-fd"
Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

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