Fix hash generation

[pedorich-n]

Since generate-hashes migrated to writeShellApplication, the hash generation jobs failed. Mainly because snapshot branch doesn't have Packages files anymore.

writeShellApplication sets these flags for the script:

set -o errexit
set -o nounset
set -o pipefail

And since NixOS/nixpkgs#280592 they are configurable. So we can remove errexit, but because this repo's nixpkgs was quite old (more than a year) I had to update it first.

Disclaimer: this doesn't fix the builds, only makes the script not fail on errors, allowing to update hashes for OpenWRT =<24

[astro]

Thank you!