fix(CI): more flexible fpm install

Author: rouson

.github/workflows/build.yml

@@ -65,7 +65,13 @@ jobs:
           ln -sf /opt/homebrew/opt/llvm@${{ matrix.compiler-version }}/bin/flang \
             /opt/homebrew/bin/flang
 
-      - name: Install fpm
+      - name: Install fpm (macOS, from source)
+        if: runner.os == 'macOS'
+        run: |
+          brew install fpm
+
+      - name: Install fpm (Linux, from source)
+        if: runner.os == 'Linux'
         uses: fortran-lang/setup-fpm@v7
         with:
           fpm-version: 'v0.13.0'