Checking parent directories for .age-recipients file stops at $PREFIX

It seems that only the `$PASSAGE_DIR` folder is checked for the `.age-recipients` file. That, or I am misunderstanding the documentation:

> For encryption, the nearest .age-recipients file (that is, the one in the same
directory as the secret, or in the closest parent) is used with the -R age CLI
option.

For the current version (`set_age_recipients()`):
```
local current="$PREFIX/$1"
# should prevent traversal above $PREFIX
# vvvvvvvvvvvvvvvvvvvvv
while [[ $current != "$PREFIX" && ! -f $current/.age-recipients ]]; do
current="${current%/*}"
done
current="$current/.age-recipients"
```

Where `PREFIX="${PASSAGE_DIR:-$HOME/.passage/store}"`, seems to prevent passage from traversing upwards. Removing the first condition leads the program to act as I would expect. There may be security implications to allowing an infinite upwards traversal in search for recipients, so the fix may be a documentation edit.

Additionally, there is no warning when the identity key fallback is used; this may lead to an unfortunate UX where additional recipients are not used to encrypt, and a "recovery" key would fail to recover the data.

Thank you for your work.