adds macos-15-intel to build matrix (#389)

Author: tisba

.github/workflows/ci.yml

@@ -16,6 +16,7 @@ jobs:
         os:
           - "macos-14" # arm64
           - "macos-15" # arm64
+          - "macos-15-intel"
           - "ubuntu-24.04"
         ruby:
           - "truffleruby+graalvm"
@@ -55,7 +56,8 @@ jobs:
       matrix:
         os:
           - "macos-14" # arm64
-          - "macos-15"
+          - "macos-15" # arm64
+          - "macos-15-intel"
         ruby:
           - "ruby-3.1"
           - "ruby-3.2"