efivar: patch build system for reproducibility

[delroth]

Motivation for this change

Make efivar build reproducible. The patch has been submitted upstream.

nix-build --check -A efivar now passes.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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 nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @grahamc (r13y)

[mmahut]

@GrahamcOfBorg build efivar

[grahamc]

Great work! Nice to get r13y another step forward :D

Two notes (for both @delroth and @mmahut):

1. generally we should use fetchpatch over fetchurl for patches, as it makes the patches a bit more stable

2. these pull request .patch URLs are quite unstable. As the PR branch changes, or the branch it is merged in to changes, this patch will change contents and the hash will become invalid. In this case fetching the commit's patch directly (https://github.com/delroth/efivar/commit/b81e2f7d72d79b941f71962819606b802e6e835c.patch) would be better.