Fix STD build failing for target_os = "espidf"
A regression from rust-lang/rust#142938
cc `@lolbinarycat`
cc `@ibraheemdev`
ESP-IDF (and a few other embedded Tier-3 systems) is considered `cfg(unix)`, but it does not have the `O_NOFOLLOW` flag because neither of its three supported filesystems (FATFS, LitteLF and Spiffs) has symbolic links in the first place.
What this fix does is to keep the `set_permissions_nofollow` method available and non-failing for ESP-IDF, but it behaves as if no `O_NONFOLLOW` was set. This should be fine as there is nothing to follow in the first place, as there are no symbolic links there.
EDIT: Also added the same fix for Horizon, as requested by `@Meziu.`
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