Point to a docker image that is in the right format in testsuite.yml

The container initialization dies with the following error:

    [DEPRECATION NOTICE] Docker Image Format v1 and Docker Image
    manifest version 2, schema 1 support is disabled by default and
    will be removed in an upcoming release. Suggest the author of
    docker.io/library/perl:5.12 to upgrade the image to the OCI Format
    or Docker Image manifest v2, schema

The `-buster` images seem to be in the expected format.

.github/workflows/testsuite.yml

@@ -49,13 +49,13 @@ jobs:
             "perl:5.26",
             "perl:5.24",
             "perl:5.22",
-            "perl:5.20",
-            "perl:5.18",
-            "perl:5.16",
-            "perl:5.14",
-            "perl:5.12",
-            "perl:5.10",
-            "perl:5.8",
+            "perl:5.20-buster",
+            "perl:5.18-buster",
+            "perl:5.16-buster",
+            "perl:5.14-buster",
+            "perl:5.12-buster",
+            "perl:5.10-buster",
+            "perl:5.8-buster",
             "perldocker/perl:5.6"
           ]