[Fix] `nvm_download_artifact`: fix error propagation from subshells

`return` inside `(...)` subshells only exits the subshell, not the calling function.
Errors in mkdir, download, and checksum verification were silently ignored.
Use `{ ...; }` brace groups instead.

Bug introduced in https://github.com/nvm-sh/nvm/commit/ba3ad8e460b88aab281d203276b354ce3c9df52f.