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

Makefile

@@ -1,6 +1,8 @@
-.PHONY: install setup uninstall \
-        test build build-no-cache build-no-cache-all build-missing delete-volumes purge \
-        test-unit test-e2e test-integration test-env-virtual test-env-nix
+.PHONY: install uninstall \
+        build build-no-cache build-no-cache-all build-missing \
+		delete-volumes purge \
+        test test-unit test-e2e test-integration test-env-virtual test-env-nix \
+		setup setup-venv setup-nix
 
 # Distro
 # Options: arch debian ubuntu fedora centos
@@ -30,11 +32,27 @@ TEST_PATTERN	:= test_*.py
 export TEST_PATTERN
 
 # ------------------------------------------------------------
-# PKGMGR setup (developer wrapper -> scripts/installation/main.sh)
+# System install
 # ------------------------------------------------------------
-setup:
+install:
+	@echo "Building and installing distro-native package-manager for this system..."
 	@bash scripts/installation/main.sh
 
+# ------------------------------------------------------------
+# PKGMGR setup
+# ------------------------------------------------------------
+
+# Default: keep current auto-detection behavior
+setup: setup-nix setup-venv
+
+# Explicit: developer setup (Python venv + shell RC + main.py install)
+setup-venv:
+	@bash scripts/setup/venv.sh
+
+# Explicit: Nix shell mode (no venv, no RC changes)
+setup-nix:
+	@bash scripts/setup/nix.sh
+
 # ------------------------------------------------------------
 # Docker build targets (delegated to scripts/build)
 # ------------------------------------------------------------
@@ -85,13 +103,6 @@ delete-volumes:
 
 purge: delete-volumes build-no-cache
 
-# ------------------------------------------------------------
-# System install (native packages, calls scripts/installation/run-package.sh)
-# ------------------------------------------------------------
-install:
-	@echo "Building and installing distro-native package-manager for this system..."
-	@bash scripts/installation/run-package.sh
-
 # ------------------------------------------------------------
 # Uninstall target
 # ------------------------------------------------------------