Fix test scripts: ensure default distro and always run via bash

- Remove Makefile inline variable export (distro=arch) and invoke scripts via bash
- Add robust default in test-unit.sh and test-integration.sh:
    : "${distro:=arch}"
- Prevent "unbound variable" errors under `set -u` when no distro is provided

scripts/test/test-integration.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+: "${distro:=arch}"
+
 echo "============================================================"
 echo ">>> Running INTEGRATION tests in ${distro} container"
 echo "============================================================"

scripts/test/test-unit.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+: "${distro:=arch}"
+
 echo "============================================================"
 echo ">>> Running UNIT tests in ${distro} container"
 echo "============================================================"