ci: instruct linker to use correct libc++

Author: hmelder

.github/workflows/weekly.yml

@@ -75,7 +75,8 @@ jobs:
             -DCMAKE_ASM_COMPILER=clang-${LLVM_VERSION} \
             -DCMAKE_CXX_COMPILER=clang++-${LLVM_VERSION} \
             -DCMAKE_OBJCXX_COMPILER=clang++-${LLVM_VERSION} \
-            -DCMAKE_CXX_FLAGS="-stdlib=${{matrix.cxxlib}}"
+            -DCMAKE_CXX_FLAGS="-stdlib=${{matrix.cxxlib}}" \
+            -DCMAKE_EXE_LINKER_FLAGS="-stdlib=${{matrix.cxxlib}} $ADDITIONAL_LINKER_FLAGS"
         # Build with a nice ninja status line
       - name: Build
         working-directory: ${{github.workspace}}/build