tachyon: disable threading on aarch64 to avoid sage test hangs

[collares]

Motivation for this change

Something changed on master between March 11th and March 20th and Sage tests that use Tachyon for 3D graphics started hanging on aarch64 (the links are to succeeding and failing ofBorg testruns). I don't have an aarch64 machine so it's hard for me to bisect the problem, but disabling multithreading makes the bug go away.

@timokau, is this an acceptable solution?

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.

[collares]

@ofborg build sage

[collares]

@timokau I am not in a hurry at all to get this merged as I don't even have access to an aarch64 system, but I thought I'd send a "this is ready (for feedback, at least)" ping just because you probably got a billion emails from my force pushes while this PR was still a draft. This fixes Sage aarch64 tests on ofBorg.

[timokau]

I tried to bisect the issue, but I already got test timeouts on aarch64 right after the sage 9.2 update (#105615). The aarch64 support of sage on nix was never exactly first class, so this might go back even further.

Thank you for tracking it down. I left a comment requesting some "breadcrumbs" for future adventurers. After that this PR should be good to go.