test: share persistent Nix store across all test containers

This commit adds the `pkgmgr_nix_store` volume mount (`/nix`) to all test
runners (unit, integration, container sanity checks, and E2E tests).

Previously only the Arch-based E2E container mounted a persistent `/nix`
store, causing all other distros (Debian, Ubuntu, Fedora, CentOS, etc.)
to download the entire Nix closure repeatedly during test runs.

Changes:
- Add `-v pkgmgr_nix_store:/nix` to:
  - scripts/test/test-container.sh
  - scripts/test/test-e2e.sh (remove Arch-only condition)
  - scripts/test/test-unit.sh
  - scripts/test/test-integration.sh
- Ensures all test containers reuse the same Nix store.

Benefits:
- Significantly faster test execution after the first run.
- Prevents redundant downloads from cache.nixos.org.
- Ensures consistent Nix environments across all test distros.

No functional changes to pkgmgr itself; only test infrastructure improved.

https://chatgpt.com/share/693890f5-2f54-800f-b47e-1925da85b434

scripts/test/test-container.sh

@@ -19,6 +19,7 @@ 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 "$(pwd):/src" \
             -v "pkgmgr_nix_cache:/root/.cache/nix" \
             "$IMAGE" 2>&1); then

scripts/test/test-e2e.sh

@@ -8,14 +8,9 @@ for distro in $DISTROS; do
   echo ">>> Running E2E tests: $distro"
   echo "============================================================"
 
-  MOUNT_NIX=""
-  if [[ "$distro" == "arch" ]]; then
-    MOUNT_NIX="-v pkgmgr_nix_store:/nix"
-  fi
-
   docker run --rm \
     -v "$(pwd):/src" \
-    $MOUNT_NIX \
+    -v pkgmgr_nix_store:/nix \
     -v "pkgmgr_nix_cache:/root/.cache/nix" \
     -e PKGMGR_DEV=1 \
     -e TEST_PATTERN="${TEST_PATTERN}" \

scripts/test/test-integration.sh

@@ -7,6 +7,7 @@ echo "============================================================"
 
 docker run --rm \
   -v "$(pwd):/src" \
+  -v pkgmgr_nix_store:/nix \
   -v "pkgmgr_nix_cache:/root/.cache/nix" \
   --workdir /src \
   -e PKGMGR_DEV=1 \

scripts/test/test-unit.sh

@@ -8,6 +8,7 @@ echo "============================================================"
 docker run --rm \
   -v "$(pwd):/src" \
   -v "pkgmgr_nix_cache:/root/.cache/nix" \
+  -v pkgmgr_nix_store:/nix \
   --workdir /src \
   -e PKGMGR_DEV=1 \
   -e TEST_PATTERN="${TEST_PATTERN}" \