CI: Add macOS 26 coverage

Author: bonachea

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ macos-13, macos-14, macos-15, macos-15-intel, ubuntu-24.04 ]
+        os: [ macos-13, macos-14, macos-15, macos-15-intel, macos-26, ubuntu-24.04 ]
         compiler: [ gfortran ]
         version: [ 13, 14, 15 ]
         network: [ smp ]