GitHub CI: Only run under Windows.

Author: trwyant

.github/workflows/ci.yml

@@ -30,19 +30,13 @@ jobs:
       matrix:
 
         # OS environments under which the job runs.
-        runner: [ubuntu-latest, macos-latest, windows-latest]
+        runner: [windows-latest]
         # Version of Perl to run. This specifies the most-recent Perl 5.
         perl: [ '5' ]
 
         # Add minimum Perl versions, which differ among operating
         # systems
         include:
-          - runner: ubuntu-latest
-            # v5.8.8 is the earliest known to work
-            perl: '5.8.8'
-          - runner: macos-latest
-            # v5.8.8 is the earliest known to work
-            perl: '5.8.8'
           - runner: windows-latest
             # v5.26.0 is the earliest known to work
             perl: '5.26.0'