tests/run.sh: Fix /build conflict when building in a sandbox
tests/run.sh: Fix /build conflict when building in a sandbox

https://hydra.nixos.org/build/64519355

peti/nix,dezgeg/nix,zimbatm/nix,rycee/nix,dtzWill/nix,vcunat/nix,rycee/nix,shlevy/nix,shlevy/nix,ehmry/nix,zimbatm/nix,NixOS/nix,peti/nix,ehmry/nix,zimbatm/nix,ehmry/nix,layus/nix,layus/nix,layus/nix,NixOS/nix,dtzWill/nix,shlevy/nix,dtzWill/nix,pikajude/nix,rycee/nix,dtzWill/nix,shlevy/nix,layus/nix,shlevy/nix,NixOS/nix,NixOS/nix,rycee/nix,pikajude/nix,zimbatm/nix,peti/nix,dezgeg/nix,dezgeg/nix,vcunat/nix,NixOS/nix,ehmry/nix,dezgeg/nix,layus/nix,peti/nix,peti/nix,vcunat/nix,dezgeg/nix,vcunat/nix,pikajude/nix,ehmry/nix,ehmry/nix,dtzWill/nix,rycee/nix