[Fix] `nvm_get_arch`: suppress error output if `/sbin` does not exist
[Fix] `nvm_get_arch`: suppress error output if `/sbin` does not exist

Introduced by #2469

PeterDaveHello/nvm,creationix/nvm,ljharb/nvm,ljharb/nvm,PeterDaveHello/nvm,creationix/nvm