chore: add script to update vendorSha256 in Nix flake

[jakubgs]

Background

Currently it is necessary to update manually the vendorSha256 in flake.nix:

go-waku/flake.nix

Lines 31 to 32 in 150ade6

	# FIXME: This needs to be manually changed when updating modules.
	vendorSha256 = "sha256-4xChSKAkwwrFp5/ZMnhtvsR4drVfw1cLE3YXwVHeW0A=";

Which requires building the node and getting correct SHA256 from the error.
It would be better if we could run a script to do this for us.

Details

I researched this a bit before and started a thread on Nix Discourse:
https://discourse.nixos.org/t/how-should-i-build-a-go-package-from-local-source/19490/11

Which led me to nix-prefetch tool, but it appears to lack support for flakes:

• Add flake support msteen/nix-prefetch#35
• src/main.sh: Add --experimental-features and --extra-experimental-features support msteen/nix-prefetch#34

Because when I try to use it nothing happens:

nix-prefetch --option extra-experimental-features flakes \
  '{ sha256 }: (builtins.getFlake (toString ./.)).packages.x86_64-linux.node.go-modules.overrideAttrs (_: { vendorSha256 = sha256; })'

But it is known to work for non-Flake derivations, here's an example in ``:

newvendorSha256="$(NIX_PATH="nixpkgs=https://github.com/nixos/nixpkgs/archive/nixos-unstable.tar.gz" \
  nix-prefetch \
    "{ sha256 }: let p=(import ./packages.nix).${upattr}; in p.go-modules.overrideAttrs (_: { vendorSha256 = sha256; })")"
sed -i "s|${vendorSha256}|${newvendorSha256}|" "${metadata}"

https://github.com/colemickens/nixpkgs-wayland/blob/c9c4a80715557caf57f403038a267f3c20859424/update.sh#L87