Fix shellcheck error in lean/helpers/build

Refactor the build script to follow the pattern used by other ecosystems
(like Julia). The script now:
- Requires DEPENDABOT_NATIVE_HELPERS_PATH to be set (fails early if not)
- Properly uses helpers_dir and install_dir variables throughout
- Removes the unused variable that triggered SC2034

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jessealama

lean/helpers/build

@@ -4,7 +4,15 @@
 
 set -e
 
-install_dir="${DEPENDABOT_NATIVE_HELPERS_PATH:-$(dirname "$0")}"
+if [ -z "$DEPENDABOT_NATIVE_HELPERS_PATH" ]; then
+  echo "Unable to build, DEPENDABOT_NATIVE_HELPERS_PATH is not set"
+  exit 1
+fi
+
+helpers_dir=$(cd -P "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+install_dir="$DEPENDABOT_NATIVE_HELPERS_PATH/lean"
+
+mkdir -p "$install_dir"
 
 # Install elan if not already installed
 if ! command -v elan &> /dev/null; then
@@ -21,12 +29,7 @@ else
 fi
 
 # Copy helper files to install directory
-if [ -n "$DEPENDABOT_NATIVE_HELPERS_PATH" ]; then
-  lean_install_dir="$DEPENDABOT_NATIVE_HELPERS_PATH/lean"
-  mkdir -p "$lean_install_dir"
-  cp -r "$(dirname "$0")"/* "$lean_install_dir/"
-  chmod +x "$lean_install_dir/run.sh"
-  echo "Lean helpers installed to $lean_install_dir"
-fi
+cp -r "$helpers_dir"/* "$install_dir/"
+chmod +x "$install_dir/run.sh"
 
-echo "Lean native helpers build complete"
+echo "Lean helpers installed to $install_dir"