Allow Elixir install for non-Jenkins users

[mattbaileyuk]

In #73 there was a change to install Hex as part of installing Elixir. There is then a step to chown the home directory, presumably because of files added by that install.

However, this broke our pipeline because we use install-elixir.sh when building a container image (so in a Dockerfile) in our pipeline, and so the user running the script is not jenkins.

So I've updated the script to support other users running the script, testing the fork in our pipeline.

[nickva]

I am not sure if this was running as jenkins or as root. It might have been as root then whoami, won't work. Maybe a CI_USER or similar variable might work here.

This might not be an issue any longer after apache/couchdb#5250. If this gets a bit tricky to fix we can probably just skip installing hex altogether and let make clean || true do it's thing on failure of make clean.

[mattbaileyuk]

In our case it was running as root rather than jenkins which is the cause of the problems (we don't even have a jenkins user when running this script, so chown fails). But with this change it's fine as I believe whoami does always return "root" when you're running as root (and even when running sudo whoami as any user)