fix(runtime): use correct archive locations (e.g. Hex) for bundled Elixir

[soundmonster]

Set MIX_ARCHIVES explicitly to make sure to override any existing setting, e.g. from asdf or mise.

What happens on main currently is that hex is installed into whatever the environment variable MIX_ARCHIVES points at. The var is set by e.g. mise. So if the "regular" project setup is running on Elixir < 1.17, its installation of Hex will be overridden by NextLS and it stops working after that.

This patch makes sure to set MIX_ARCHIVES explicitly so the hex archive ends up in the right place.

[soundmonster]

After a closer look I realized that installing into the right folder is only one part of it, we also need to make sure we call Hex from that place going forward. Hence the second commit that sets MIX_ARCHIVES in all places where MIX_HOME is being set. This makes sure to override what asdf/mise have set up, so that hex works with bundled mix even if it's not installed outside of the LSP.

[mhanberg]

Thanks!