github: fix tests by switching to buster images for old perls

See Perl/docker-perl#161 for more information.

Otherwise we get:

    /usr/bin/docker pull perl:5.20
      5.20: Pulling from library/perl
      [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:5.20 to upgrade the image to the OCI Format or Docker Image manifest v2, schema 2. More information at https://docs.docker.com/go/deprecated-image-specs/
      Warning: Docker pull failed with exit code 1, back off 7.724 seconds before retry.
      /usr/bin/docker pull perl:5.20
      5.20: Pulling from library/perl
      [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:5.20 to upgrade the image to the OCI Format or Docker Image manifest v2, schema 2. More information at https://docs.docker.com/go/deprecated-image-specs/
      Warning: Docker pull failed with exit code 1, back off 9.064 seconds before retry.
      /usr/bin/docker pull perl:5.20
      5.20: Pulling from library/perl
      [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:5.20 to upgrade the image to the OCI Format or Docker Image manifest v2, schema 2. More information at https://docs.docker.com/go/deprecated-image-specs/
      Error: Docker pull failed with exit code 1

Author: mauke

.github/workflows/testsuite.yml

@@ -54,13 +54,13 @@ jobs:
             "5.26",
             "5.24",
             "5.22",
-            "5.20",
-            "5.18",
-            "5.16",
-            "5.14",
-            "5.12",
-            "5.10",
-            "5.8",
+            "5.20-buster",
+            "5.18-buster",
+            "5.16-buster",
+            "5.14-buster",
+            "5.12-buster",
+            "5.10-buster",
+            "5.8-buster",
           ]
 
     container: