Use libc++ in Ubuntu Clang Build (facebook#1440)

Summary:
An ubuntu-latest image update caused build failures, where GTest importing chrono headers caused an error inside of stdlib++ when being compiled by Clang.

This switches to explicitly asking for libc++ (LLVM stdlib) in the reference Ubuntu Clang build instead of stdlibc++ (GCC stdlib).

We don’t force this in CMake logic, to not force a specific stdlib for users compiling Yoga alongside other libraries. E.g. facebookexperimental/libunifex#86

Pull Request resolved: facebook#1440

Reviewed By: joevilches

Differential Revision: D50790965

Pulled By: NickGerleman

fbshipit-source-id: fe525cfb4385a9a5ae6a02fcbc6decad0e07ff6e

.github/actions/setup-cpp/action.yml

@@ -18,6 +18,8 @@ runs:
       run: |
         echo "CC=/usr/bin/clang" >> $GITHUB_ENV
         echo "CXX=/usr/bin/clang++" >> $GITHUB_ENV
+        echo "CXXFLAGS=-stdlib=libc++" >> $GITHUB_ENV
+        echo "LDFLAGS=-stdlib=libc++" >> $GITHUB_ENV
 
     - name: Set GCC as compiler
       if: ${{ inputs.toolchain == 'GCC' }}