Ensure install.sh exits if either of OTP or Elixir fails to install

[zachallaun]

This script uses set -e to immediately exit the program if any commands return non-zero, but wait without arguments does not reliably return with the exit code of a failed subprocess. (Exactly which exit code is returned depends, I believe, on the order in which the subprocesses exit.)

By waiting on both pids explicitly, we ensure that the script exits if either OTP or Elixir fails to install. In the case of #1781, this change makes it so that the script halts after the curl fails to download OTP master instead of carrying on as if nothing failed.

[zachallaun]

Note that I didn't change install.bat because it does not install in parallel and so doesn't have this issue.

[josevalim]

cc @wojtekmach

[josevalim]

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