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

.github/workflows/test-container.yml

@@ -7,6 +7,10 @@ jobs:
   test-container:
     runs-on: ubuntu-latest
     timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        distro: [arch, debian, ubuntu, fedora, centos]
 
     steps:
       - name: Checkout repository
@@ -15,5 +19,7 @@ jobs:
       - name: Show Docker version
         run: docker version
 
-      - name: Run container tests
-        run: make test-container
+      - name: Run container tests (${{ matrix.distro }})
+        run: |
+          set -euo pipefail
+          distro="${{ matrix.distro }}" make test-container