Fix CI: use libc++ on linux with clang

Workarounds actions/runner-images#8659

.github/workflows/on_pull_request.yml

@@ -106,14 +106,16 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install Dependencies
         run: |
-          sudo apt install -y clang lld
+          sudo apt install -y clang lld "libc++-dev"
       - name: Configure
         run: |
           set -x
           mkdir Clang
           cd Clang
           export CC=$(which clang)
           export CXX=$(which clang++)
+          export CXXFLAGS=$(-stdlib=libc++)
+          export LDFLAGS=$(-stdlib=libc++)
           cmake -DCMAKE_CXX_STANDARD=20 ..
       - name: Build
         run: |