blackbox: only doCheck on x86_64-linux

[Artturin]

Motivation for this change

Closes #134445

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages 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/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[Artturin]

Fails on aarch64-linux too so disabling it

blackbox> ========== Decrypting new/changed files: START
blackbox> Please enter the passphrase to unlock the OpenPGP secret key:
blackbox> "Bob Example (my password is the lowercase letter b) <bob@example.com>"
blackbox> 3072-bit RSA key, ID 4D88605DA7B4752D,
blackbox> created 2021-09-07 (main key ID 58065DE4EAD1E8E1).
blackbox> Passphrase: b
blackbox> gpg-agent[709]: failed to unprotect the secret key: Timeout
blackbox> gpg-agent[709]: failed to read the secret key
blackbox> gpg-agent[709]: command 'PKDECRYPT' failed: Timeout <Pinentry>
blackbox> gpg: decryption failed: No secret key
blackbox> gpg-agent[701]: socket file has been removed - shutting down
blackbox> gpg-agent[701]: gpg-agent (GnuPG) 2.2.27 stopped
blackbox> gpg-agent[709]: socket file has been removed - shutting down
blackbox> gpg-agent[709]: gpg-agent (GnuPG) 2.2.27 stopped
blackbox> make: *** [Makefile:146: confidence] Error 1