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/docker/entry.sh

@@ -68,16 +68,10 @@ cd /src
 # ---------------------------------------------------------------------------
 # DEV mode: rebuild package-manager from the mounted /src tree
 # ---------------------------------------------------------------------------
-if [[ "${PKGMGR_DEV:-0}" == "1" ]]; then
-  echo "[docker] DEV mode enabled (PKGMGR_DEV=1)"
-  echo "[docker] Rebuilding package-manager from /src via scripts/installation/run-package.sh..."
-
-  if [[ -x scripts/installation/run-package.sh ]]; then
-    bash scripts/installation/run-package.sh
-  else
-    echo "[docker] ERROR: scripts/installation/run-package.sh not found or not executable"
-    exit 1
-  fi
+if [[ "${REINSTALL_PKGMGR:-0}" == "1" ]]; then
+  echo "[docker] DEV mode enabled (REINSTALL_PKGMGR=1)"
+  echo "[docker] Rebuilding package-manager from /src via scripts/installation/package.sh..."
+  bash scripts/installation/package.sh || exit 1
 fi
 
 # ---------------------------------------------------------------------------