CI-unixish.yml: added macos-15 (danmar#407)

Author: firewave

.github/workflows/CI-unixish.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         compiler: [clang++, g++]
-        os: [ubuntu-22.04, ubuntu-24.04, macos-13, macos-14]
+        os: [ubuntu-22.04, ubuntu-24.04, macos-13, macos-14, macos-15]
       fail-fast: false
 
     runs-on: ${{ matrix.os }}