test: isolate Nix store/cache per distro to fix cross-distro manifest conflicts

- Replace shared Nix volumes with distro-specific volumes
  (pkgmgr_nix_store_<distro>, pkgmgr_nix_cache_<distro>)
- Prevent incompatible profile manifest versions between Ubuntu and Debian
- Update all test scripts (unit, integration, container, e2e)
- Remove unused global Nix volume variables from Makefile
- Improve consistency of test-e2e.sh formatting and environment handling
- Add Git safe.directory configuration for mounted /src to avoid ownership warnings

scripts/test/test-container.sh

@@ -19,9 +19,9 @@ for distro in $DISTROS; do
     # Run the command and capture the output
     if OUTPUT=$(docker run --rm \
             -e PKGMGR_DEV=1 \
-            -v pkgmgr_nix_store:/nix \
+            -v pkgmgr_nix_store_${distro}:/nix \
             -v "$(pwd):/src" \
-            -v "pkgmgr_nix_cache:/root/.cache/nix" \
+            -v "pkgmgr_nix_cache_${distro}:/root/.cache/nix" \
             "$IMAGE" 2>&1); then
         echo "$OUTPUT"
         echo