fix(expert): omit scrubbed env vars instead of setting to empty string

[jollyjerr]

Hello expert team 👋🏼 this PR fixes an issue I have been encountering after this pr using this setup:

ide: neovim v0.12.0
expert install method: build from source with `just release`
language version manager: mise

When launching neovim in a project directory, expert will crash with:

LSP[expert] [my-project] Failed to start engine my-project::24676988: {:error, "Failed to determine Elixir/OTP runtime for project: Runtime terminating during boot ({'cannot get bootfile','/Users/me/cod
e/opensource/expert/apps/expert/_build/prod/rel/plain/bin/start.boot'})\r\n\r\nCrash dump is being written to: erl_crash.dump...done (status 1)"}

I believe this is happening because we are passing ROOTDIR="" to erlexec here so we are skipping this logic

Example of bug

ERLEXEC=~/.local/share/mise/installs/erlang/26.2.5.15/erts-14.2.5.11/bin/erlexec
BINDIR=~/.local/share/mise/installs/erlang/26.2.5.15/erts-14.2.5.11/bin

env -i HOME="$HOME" PATH="$PATH" BINDIR="$BINDIR" "$ERLEXEC" -noshell -eval 'halt().' 2>&1; echo "exit: $?"

# prints exit: 0

env -i HOME="$HOME" PATH="$PATH" ROOTDIR="" BINDIR="$BINDIR" "$ERLEXEC" -noshell -eval 'halt().' 2>&1; echo "exit: $?"

# prints
# Runtime terminating during boot ({'cannot get bootfile','/bin/start.boot'})
# Crash dump is being written to: erl_crash.dump...done
# exit: 1

By omitting @scrubbed_env_vars entirely, I was able to get expert working again with my setup.

I believe this will also be the fix for #637