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/test/test-container.sh

@@ -1,41 +1,30 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-echo "============================================================"
-echo ">>> Running sanity test: verifying test containers start"
-echo "============================================================"
-
-for distro in $DISTROS; do
-    IMAGE="package-manager-test-$distro"
-
-    echo
-    echo "------------------------------------------------------------"
-    echo ">>> Testing container: $IMAGE"
-    echo "------------------------------------------------------------"
-
-    echo "[test-container] Running: docker run --rm --entrypoint pkgmgr $IMAGE --help"
-    echo
-
-    # Run the command and capture the output
-    if OUTPUT=$(docker run --rm \
-            -e PKGMGR_DEV=1 \
-            -v pkgmgr_nix_store_${distro}:/nix \
-            -v "$(pwd):/src" \
-            -v "pkgmgr_nix_cache_${distro}:/root/.cache/nix" \
-            "$IMAGE" 2>&1); then
-        echo "$OUTPUT"
-        echo
-        echo "[test-container] SUCCESS: $IMAGE responded to 'pkgmgr --help'"
-
-    else
-        echo "$OUTPUT"
-        echo
-        echo "[test-container] ERROR: $IMAGE failed to run 'pkgmgr --help'"
-        exit 1
-    fi
-done
+IMAGE="package-manager-test-$distro"
 
 echo
-echo "============================================================"
-echo ">>> All containers passed the sanity check"
-echo "============================================================"
+echo "------------------------------------------------------------"
+echo ">>> Testing container: $IMAGE"
+echo "------------------------------------------------------------"
+
+echo "[test-container] Running: docker run --rm --entrypoint pkgmgr $IMAGE --help"
+echo
+
+# Run the command and capture the output
+if OUTPUT=$(docker run --rm \
+        -e PKGMGR_DEV=1 \
+        -v pkgmgr_nix_store_${distro}:/nix \
+        -v "$(pwd):/src" \
+        -v "pkgmgr_nix_cache_${distro}:/root/.cache/nix" \
+        "$IMAGE" 2>&1); then
+    echo "$OUTPUT"
+    echo
+    echo "[test-container] SUCCESS: $IMAGE responded to 'pkgmgr --help'"
+
+else
+    echo "$OUTPUT"
+    echo
+    echo "[test-container] ERROR: $IMAGE failed to run 'pkgmgr --help'"
+    exit 1
+fi