In newer versions, instead of using $PWD to locate its ressource files,
Refind now refers to the dir containing $0.
This causes runtime errors due to missing ressources.

In lieu of a wrapper binary, we now simply patch the variable RefindDir
which stores the path to the ressource dir.

Here's a short script (run as root) to help testing this PR.
To see the runtime error that was fixed, replace the call to refind-install with the current Nixpkgs version of refind.

## Summary: Create temp device, install refind, delete device

# Create temporary loop device
truncate -s 10M /tmp/buf
tmpdev=$(losetup --find --show /tmp/buf)
mkfs.fat $tmpdev

# Run refind 
$(nix-build --no-out-link '<nixpkgs-dev>' -A refind)/bin/refind-install --usedefault $tmpdev --alldrivers
# Show contents of installation
$(nix-build --no-out-link '<nixpkgs>' -A tree)/bin/tree /tmp/refind_install

# Remove device
umount $tmpdev
losetup --detach $tmpdev
rm /tmp/buf

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.