ci: fix libc++ std.cppm path for CMake import_std on Clang-21+

CMake detects the std.cppm location relative to where libc++.so is linked
(/usr/lib/x86_64-linux-gnu/), producing a normalised path that resolves to
/lib/share/libc++/v1/std.cppm. On Debian/Ubuntu the actual file lives under
/usr/lib/llvm-N/share/libc++/v1/, so every try_compile inner project fails to
generate because the source file does not exist at the expected path.

Create a symlink /lib/share/libc++ -> /usr/lib/llvm-N/share/libc++ before
the build step so CMake can find the module sources. Only needed when
import_std is enabled (the trigger for CMake's module-path detection).

Author: mpusz

.github/workflows/ci-freestanding.yml

@@ -118,6 +118,18 @@ jobs:
         shell: bash
         run: |
           sudo apt install -y libc++-${{ matrix.toolchain.compiler.version }}-dev libc++abi-${{ matrix.toolchain.compiler.version }}-dev libunwind-${{ matrix.toolchain.compiler.version }}-dev
+      - name: Fix libc++ std.cppm path for CMake
+        # CMake detects std.cppm relative to where libc++.so is linked
+        # (/usr/lib/x86_64-linux-gnu/), producing a path like
+        # /lib/x86_64-linux-gnu/../share/libc++/v1/std.cppm which resolves to
+        # /lib/share/libc++/v1/std.cppm. On Ubuntu the actual file lives under
+        # /usr/lib/llvm-N/share/libc++/v1/. Create the expected symlink so that
+        # CMake's try_compile inner projects can find it.
+        if: matrix.import_std == 'True' && matrix.toolchain.compiler.type == 'CLANG'
+        shell: bash
+        run: |
+          sudo mkdir -p /lib/share
+          sudo ln -sfn /usr/lib/llvm-${{ matrix.toolchain.compiler.version }}/share/libc++ /lib/share/libc++
       - name: Set up Python
         uses: actions/setup-python@v5
         with: