Use release-specific tags for older perl images - Perl/docker-perl#161

Author: mohawk2

.github/workflows/ci.yml

@@ -13,9 +13,9 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        perl-version: ['5.8', '5.14', '5.20']
+        perl-version: ['5.8-buster', '5.14-buster', '5.20-buster']
         include:
-          - perl-version: '5.30'
+          - perl-version: 'latest'
             os: ubuntu-latest
             release-test: true
             coverage: true