Fix perl docker version tag

Author: Duncan Ferguson

.github/workflows/dzil_tester.yml

@@ -13,20 +13,19 @@ jobs:
       fail-fast: false
       matrix:
         perl-version:
-          - latest
-          - 5.40
-          - 5.38
-          - 5.36
-          - 5.34
-          - 5.32
-          - 5.30
-          - 5.28
-          - 5.26
-          - 5.24
-          - 5.22
-          - 5.20
-          - 5.18
-          - 5.16
+          - '5.40'
+          - '5.38'
+          - '5.36'
+          - '5.34'
+          - '5.32'
+          - '5.30'
+          - '5.28'
+          - '5.26'
+          - '5.24'
+          - '5.22'
+          - '5.20'
+          - '5.18'
+          - '5.16'
         include:
           - perl-version: '5.38'
             os: ubuntu-latest