Refactor CI distro handling and container build scripts

- Introduce a GitHub Actions matrix for `test-container` and `test-e2e`
  to run against arch, debian, ubuntu, fedora, and centos
- Run unit and integration tests only in the Arch container by passing
  `distro="arch"` via make in the corresponding workflows
- Replace the global DISTROS loop with a single `distro` variable in
  the Makefile, defaulting to `arch`, and export it for all scripts
- Update build scripts (build-image, build-image-no-cache, build-image-missing)
  to build images for the selected distro only
- Simplify test-container script to validate a single distro image using
  the `distro` environment variable
- Simplify E2E, unit, and integration test scripts to run against a
  single distro container instead of iterating over all distros

https://chatgpt.com/share/693acbba-9e30-800f-94fb-fea4489e9078

scripts/build/build-image.sh

@@ -4,13 +4,11 @@ set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 source "${SCRIPT_DIR}/resolve-base-image.sh"
 
-for distro in $DISTROS; do
-  base_image="$(resolve_base_image "$distro")"
+base_image="$(resolve_base_image "$distro")"
 
-  echo ">>> Building test image for distro '$distro' (BASE_IMAGE=$base_image)..."
+echo ">>> Building test image for distro '$distro' (BASE_IMAGE=$base_image)..."
 
-  docker build \
-    --build-arg BASE_IMAGE="$base_image" \
-    -t "package-manager-test-$distro" \
-    .
-done
+docker build \
+  --build-arg BASE_IMAGE="$base_image" \
+  -t "package-manager-test-$distro" \
+  .