scripts/bootstrap.sh: add libidn2 to bootstrap path
libidn2-dev is dependent by curl，and so libidn2 is needed add to the bootstrap path.
We are currently working on getting Alpine Linux 3.21 out. This means that merge requests that are not aimed to help with that (only affecting testing, new aports) have a lower priority at the moment. Please be patient, they will be eventually picked up.
libidn2-dev is dependent by curl，and so libidn2 is needed add to the bootstrap path.