During CI, use just 1 cpu for QEMU builds
During CI, use just 1 cpu for QEMU builds

https://github.com/cirruslabs/cirrus-ci-docs/issues/741

geofft/nix-rust,geofft/nix-rust,nix-rust/nix,asomers/nix,asomers/nix,nix-rust/nix,geofft/nix-rust