CI: get linux tests running again with buster images

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

Author: mauke

.github/workflows/linux.yml

@@ -21,12 +21,12 @@ jobs:
           # - '5.26'
           # - '5.24'
           # - '5.22'
-          # - '5.20'
-          - '5.18'
-          # - '5.16'
-          # - '5.14'
-          # - '5.12'
-          - '5.10'
+          # - '5.20-buster'
+          - '5.18-buster'
+          # - '5.16-buster'
+          # - '5.14-buster'
+          # - '5.12-buster'
+          - '5.10-buster'
     container:
       image: perl:${{ matrix.perl-version }}
     steps: