fix syntax

Author: M2-TE

.github/workflows/build.yml

@@ -49,9 +49,9 @@ jobs:
 
     env:
       VAR_CMAKE_FLAGS: |
-        ${{ matrix.env.c && format('-D CMAKE_C_COMPILER={0}', matrix.env.c) }}
-        ${{ matrix.env.cxx && format('-D CMAKE_CXX_COMPILER={0}', matrix.env.cxx) }}
-        ${{ matrix.env.cxx_flags && format('-D CMAKE_CXX_FLAGS''={0}''', matrix.env.cxx_flags) }}
+        ${{ matrix.env.c && format('-D CMAKE_C_COMPILER={0}', matrix.env.c) }} \
+        ${{ matrix.env.cxx && format('-D CMAKE_CXX_COMPILER={0}', matrix.env.cxx) }} \
+        ${{ matrix.env.cxx_flags && format('-D CMAKE_CXX_FLAGS''={0}''', matrix.env.cxx_flags) }} \
         ${{ matrix.env.linker_flags && format('-D CMAKE_EXE_LINKER_FLAGS=''{0}''', matrix.env.linker_flags) }}
       
     steps: