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/package.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+# shellcheck source=/dev/null
+source "${SCRIPT_DIR}/lib.sh"
+
+OS_ID="$(detect_os_id)"
+
+# Map Manjaro to Arch
+if [[ "${OS_ID}" == "manjaro" ]]; then
+  echo "[package] Mapping OS 'manjaro' → 'arch'"
+  OS_ID="arch"
+fi
+
+echo "[package] Detected OS: ${OS_ID}"
+
+case "${OS_ID}" in
+  arch|debian|ubuntu|fedora|centos)
+    PKG_SCRIPT="${SCRIPT_DIR}/${OS_ID}/package.sh"
+    ;;
+  *)
+    echo "[package] Unsupported OS: ${OS_ID}"
+    exit 1
+    ;;
+esac
+
+if [[ ! -f "${PKG_SCRIPT}" ]]; then
+  echo "[package] Package script not found: ${PKG_SCRIPT}"
+  exit 1
+fi
+
+echo "[package] Executing: ${PKG_SCRIPT}"
+exec bash "${PKG_SCRIPT}"