Aro and libffi should be optional, lazy dependencies

[alexrp]

graf/build.zig

Lines 103 to 107 in 662bf28

	// TODO: Aro should be a lazy dependency, but this causes HTTP problems on macOS.
	graf_mod.addImport("aro", b.dependency("aro", .{
	.target = target_opt,
	.optimize = optimize_opt,
	}).module("aro"));

Making it lazy causes CI failures: https://github.com/vezel-dev/graf/actions/runs/9533073069/job/26275888084

Upstream bug:

• invalid HTTP response: EndOfStream when fetching normal + lazy dependencies on a macos-14 GitHub Actions runner ziglang/zig#20313

[alexrp]

Applied a workaround in 662bf28.

[alexrp]

This should also be done for libffi.