ci: pull php-5.6 loader test image from Docker Hub (#4039)

The php-5.6_buster CI image is no longer published to the internal
registry (registry.ddbuild.io/ci/dd-trace-php/dd-trace-ci), so the
"Loader test on amd64 libc: [5.6, nts, buster]" job failed with
ErrImagePull/NotFound.

Keep the job and source only the 5.6 image from Docker Hub via the
ddbuild mirror (registry.ddbuild.io/images/mirror/datadog/dd-trace-ci)
through a per-matrix LOADER_IMAGE_REPO override. All other PHP versions
and the arm64 job continue to use the internal registry.

Signed-off-by: Alexandre Rulleau <alexandre.rulleau@datadoghq.com>

Author: Leiyks

.gitlab/generate-package.php

@@ -1406,12 +1406,13 @@ function appsec_image_from_tag_mapping(string $tag): string
 <?php foreach ($arch_targets as $arch): ?>
 "Loader test on <?= $arch ?> libc":
   stage: verify
-  image: "registry.ddbuild.io/ci/dd-trace-php/dd-trace-ci:php-${MAJOR_MINOR}_${CONTAINER_SUFFIX}"
+  image: "${LOADER_IMAGE_REPO}:php-${MAJOR_MINOR}_${CONTAINER_SUFFIX}"
   tags: [ "arch:$ARCH" ]
   variables:
     VALGRIND: false
     ARCH: "<?= $arch ?>"
     CONTAINER_SUFFIX: bookworm-9
+    LOADER_IMAGE_REPO: "registry.ddbuild.io/ci/dd-trace-php/dd-trace-ci"
   needs:
     - job: "package loader: [<?= $arch ?>]"
       artifacts: true
@@ -1422,6 +1423,7 @@ function appsec_image_from_tag_mapping(string $tag): string
           - "5.6"
         PHP_FLAVOUR: nts
         CONTAINER_SUFFIX: buster
+        LOADER_IMAGE_REPO: "registry.ddbuild.io/images/mirror/datadog/dd-trace-ci"
       - MAJOR_MINOR:
           - "7.0"
           - "7.1"