fix(CI): llvm -> flang

Author: rouson

.github/workflows/build.yml

@@ -45,7 +45,7 @@ jobs:
 
           # flang on Linux
           - os: ubuntu-latest
-            compiler: llvm
+            compiler: flang
             compiler-version: 21
             fpm-compiler: flang
           - os: ubuntu-latest
@@ -55,11 +55,11 @@ jobs:
 
           # flang on macOS
           - os: macos-latest
-            compiler: llvm
+            compiler: flang
             compiler-version: 21
             fpm-compiler: flang
           - os: macos-latest
-            compiler: llvm
+            compiler: flang
             compiler-version: 22
             fpm-compiler: flang