If you open the interactive terminal, can you run elixir in there? I’ve found debugging path locations from that terminal to be helpful. I install elixir and friends in a devcontainer with nix, so I’ve hit path issues a number of times.
You might also try running > Reload Window. I know you’ve rebooted, but the window reload can pick up env changes that happen after VSCode starts (a “common” issue for nix+devcontainer).
