feat(CI): add gfortran 15 on Linux

Author: rouson

.github/workflows/build.yml

@@ -24,6 +24,10 @@ jobs:
             compiler: gfortran
             compiler-version: 14
             fpm-compiler: gfortran
+          - os: ubuntu-latest
+            compiler: gfortran
+            compiler-version: 15
+            fpm-compiler: gfortran
 
           # gfortran on macOS
           - os: macos-latest