Typo in the YAML.

Author: kyle-github

.github/workflows/ci.yml

@@ -428,8 +428,8 @@ jobs:
           -DCMAKE_BUILD_TYPE=Debug \
           -DCMAKE_C_COMPILER=${{env.CMAKE_C_COMPILER}} \
           -DPTK_BUILD_TESTS=ON \
-          ${{env.CMAKE_C_FLAGS:+-DCMAKE_C_FLAGS="$CMAKE_C_FLAGS"}} \
-          ${{env.CMAKE_EXE_LINKER_FLAGS:+-DCMAKE_EXE_LINKER_FLAGS="$CMAKE_EXE_LINKER_FLAGS"}}
+          $([ -n "$CMAKE_C_FLAGS" ] && echo "-DCMAKE_C_FLAGS=$CMAKE_C_FLAGS") \
+          $([ -n "$CMAKE_EXE_LINKER_FLAGS" ] && echo "-DCMAKE_EXE_LINKER_FLAGS=$CMAKE_EXE_LINKER_FLAGS")
     
     - name: Build
       run: |