aerc: fix GOFLAGS

[zowoq]

Motivation for this change

buildGoModule doesn't pass additional GOFLAGS.

nixpkgs/pkgs/development/go-modules/generic/default.nix

Line 120 in f138879

GOFLAGS = "-mod=vendor";

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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.

[kalbasit]

@zowoq wouldn't it make more sense to add support for it in buildGoModule?

[zowoq]

This is the only package that uses GOFLAGS like this. Supporting {build,install,make}Flags, etc when we override the default phases is something I've been thinking about but that's another PR.

[kalbasit]

Fair enough!