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/setup/nix.sh

@@ -0,0 +1,9 @@
+# ------------------------------------------------------------
+# Nix shell mode: do not touch venv, only run main.py install
+# ------------------------------------------------------------
+
+echo "[setup] Nix mode enabled (NIX_ENABLED=1)."
+echo "[setup] Skipping virtualenv creation and dependency installation."
+echo "[setup] Running main.py install via system python3..."
+python3 main.py install
+echo "[setup] Setup finished (Nix mode)."