You can also use GitHub - asdf-vm/asdf: Extendable version manager with support for Ruby, Node.js, Elixir, Erlang & more

it is, but maybe a little scary. Also there are some incomplete instructions out there (missing hex and rebar eg). Nerves comes with proper instructions, I copy the relevant (non-nerves part) here:

git clone https://github.com/asdf-vm/asdf.git ~/.asdf --branch v0.10.0

# The following steps are for bash. If you’re using something else, do the
# equivalent for your shell.
echo -e '\n. $HOME/.asdf/asdf.sh' >> ~/.bashrc
echo -e '\n. $HOME/.asdf/completions/asdf.bash' >> ~/.bashrc # optional
source ~/.bashrc

asdf plugin-add erlang
asdf plugin-add elixir

# (omitting wxgtk-part as it is not needed for most)

# asdf is now setup and one can install erlang and elixir.
# asdf install erlang <TAB> gives available versions. 
# Latest releases at the time of this writing would be

asdf install erlang 25.1.1
asdf install elixir 1.14.1-otp-25

# you can have as many versions as you like installed at a time and then select them by

asdf global erlang 25.1.1
asdf global elixir 1.14.1-otp-25

# after any "asdf global xxx"  It is important to update the versions of hex and rebar used by Elixir, 
# even if you already had Elixir installed.

mix local.hex
mix local.rebar

# and most people will also want to have
mix archive.install hex phx_new

https://hexdocs.pm/nerves/installation.html#all-platforms

As a 20+ years Debian user I gave up on all 3rd party repo long time ago. You either use the packages from the distro or compile yourself. Elixir is a no-brainier: It compiles in one minute, and has no strange compile time dependency.