feat(ci): stabilize virgin Arch tests with Makefile install/setup and Nix Git safety

* Switch virgin root/user workflows to use *make install* + *make setup/setup-venv*
* Add Git *safe.directory /src* to avoid flake evaluation failures on mounted repos
* Enable Nix flake run in workflows and prepare */nix* for non-root execution
* Refactor Arch packaging to build in an isolated */tmp* directory via *aur_builder*
* Rename installer scripts (*run-** → *dependencies.sh* / *package.sh*) and adjust Docker entry + env var to *REINSTALL_PKGMGR*

https://chatgpt.com/share/693c29d9-9b28-800f-a549-5661c783d968

scripts/installation/arch/package.sh

@@ -1,30 +1,64 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-echo "[arch/package] Building Arch package (makepkg --nodeps)..."
+echo "[arch/package] Building Arch package (makepkg --nodeps) in an isolated build dir..."
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PROJECT_ROOT="$(cd "${SCRIPT_DIR}/../../.." && pwd)"
-PKG_DIR="${PROJECT_ROOT}/packaging/arch"
 
-if [[ ! -f "${PKG_DIR}/PKGBUILD" ]]; then
-  echo "[arch/package] ERROR: PKGBUILD not found in ${PKG_DIR}"
+# We must not build inside /src (mounted repo). Build in /tmp to avoid permission issues.
+BUILD_ROOT="/tmp/package-manager-arch-build"
+PKG_SRC_DIR="${PROJECT_ROOT}/packaging/arch"
+PKG_BUILD_DIR="${BUILD_ROOT}/packaging/arch"
+
+if [[ ! -f "${PKG_SRC_DIR}/PKGBUILD" ]]; then
+  echo "[arch/package] ERROR: PKGBUILD not found in ${PKG_SRC_DIR}"
   exit 1
 fi
 
-cd "${PKG_DIR}"
+echo "[arch/package] Preparing build directory: ${BUILD_ROOT}"
+rm -rf "${BUILD_ROOT}"
+mkdir -p "${BUILD_ROOT}"
 
-if id aur_builder >/dev/null 2>&1; then
-  echo "[arch/package] Using 'aur_builder' user for makepkg..."
-  chown -R aur_builder:aur_builder "${PKG_DIR}"
-  su aur_builder -c "cd '${PKG_DIR}' && rm -f package-manager-*.pkg.tar.* && makepkg --noconfirm --clean --nodeps"
-else
-  echo "[arch/package] WARNING: user 'aur_builder' not found, running makepkg as current user..."
-  rm -f package-manager-*.pkg.tar.*
-  makepkg --noconfirm --clean --nodeps
+echo "[arch/package] Syncing project sources to ${BUILD_ROOT}..."
+# Keep it simple: copy everything; adjust excludes if needed later.
+rsync -a --delete \
+  --exclude '.git' \
+  --exclude '.venv' \
+  --exclude '.venvs' \
+  --exclude '__pycache__' \
+  --exclude '*.pyc' \
+  "${PROJECT_ROOT}/" "${BUILD_ROOT}/"
+
+if [[ ! -d "${PKG_BUILD_DIR}" ]]; then
+  echo "[arch/package] ERROR: Build PKG dir missing: ${PKG_BUILD_DIR}"
+  exit 1
 fi
 
+# ------------------------------------------------------------
+# Unprivileged user for Arch package build (makepkg)
+# ------------------------------------------------------------
+if ! id aur_builder >/dev/null 2>&1; then
+  echo "[arch/package] ERROR: user 'aur_builder' not found. Run scripts/installation/arch/aur-builder-setup.sh first."
+  exit 1
+fi
+
+echo "[arch/package] Using 'aur_builder' user for makepkg..."
+chown -R aur_builder:aur_builder "${BUILD_ROOT}"
+
+echo "[arch/package] Running makepkg in: ${PKG_BUILD_DIR}"
+su aur_builder -c "cd '${PKG_BUILD_DIR}' && rm -f package-manager-*.pkg.tar.* && makepkg --noconfirm --clean --nodeps"
+
 echo "[arch/package] Installing generated Arch package..."
-pacman -U --noconfirm package-manager-*.pkg.tar.*
+pkg_path="$(find "${PKG_BUILD_DIR}" -maxdepth 1 -type f -name 'package-manager-*.pkg.tar.*' | head -n1)"
+if [[ -z "${pkg_path}" ]]; then
+  echo "[arch/package] ERROR: Built package not found in ${PKG_BUILD_DIR}"
+  exit 1
+fi
+
+pacman -U --noconfirm "${pkg_path}"
+
+echo "[arch/package] Cleanup build directory..."
+rm -rf "${BUILD_ROOT}"
 
 echo "[arch/package] Done."