If the app explicitly grants access to the host /tmp (for
instance telegram) then when this is being exposed as a symlink
in the sandbox we get an error because /tmp already exists
as a dir, which we create very early on.
It doesn't really make sense to keep /tmp as a symlink in
the sandbox anyway, so we just special case this and mount
the symlink target as /tmp.
(cherry picked from commit f28d318cc9)