Update macos images for Github Actions to use macos-15

Author: y9v

.github/workflows/nix.yml

@@ -19,10 +19,10 @@ jobs:
         platform:
           - os: darwin
             cpu: x86_64
-            base: macos-13 # always x86_64-darwin
+            base: macos-15-intel # always x86_64-darwin
           - os: darwin
             cpu: arm64
-            base: macos-14 # always arm64-darwin
+            base: macos-15 # always arm64-darwin
           - os: linux
             cpu: x86_64
             base: ubuntu-24.04 # always x86_64-linux-gnu

.github/workflows/test-macos.yaml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - macos-13
+          - macos-15
         ruby:
           - '2.5'
           - '2.6'