Fixes#2420. When using `nvm install` with an already installed
version, Bash (and potentially other shells) may emit an unbound
variable error when `set -u` is set. The presence of the error depends
on specific shell versions, but appears to happen in MacOS 11.1 (Big
Sur) using a recent Bash version installed via Homebrew (as of this
writing).
The fix accesses `ALIAS` correctly for the case where it is not set by
an `alias` option to `nvm install`.
