Explicit C++ toolchain on ubuntu-latest (#1334)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1334

Last change made me discover CMake on Ubuntu defaults to GCC. This change makes it so that we explicitly configure whether to use Clang/LLVM or GCC when building on machines with the `ubuntu-latest` image.

Reviewed By: christophpurrer

Differential Revision: D47462132

fbshipit-source-id: e81fb6d4b1740ab2913d39b6e90c52d674e5a4f2

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

@@ -1,4 +1,9 @@
 name: Setup C++ envirionment
+inputs:
+  toolchain:
+    description: Compiler toolchain to use (Clang, GCC, or MSVC)
+    required: false
+    default: 'Clang'
 
 runs:
   using: "composite"
@@ -7,6 +12,20 @@ runs:
       if: ${{ runner.os != 'Windows' }}
       uses: ./.github/actions/install-ninja
 
+    - name: Set Clang as compiler
+      if: ${{ inputs.toolchain == 'Clang' }}
+      shell: bash
+      run: |
+        echo "CC=/usr/bin/clang" >> $GITHUB_ENV
+        echo "CXX=/usr/bin/clang++" >> $GITHUB_ENV
+
+    - name: Set GCC as compiler
+      if: ${{ inputs.toolchain == 'GCC' }}
+      shell: bash
+      run: |
+        echo "CC=/usr/bin/gcc" >> $GITHUB_ENV
+        echo "CXX=/usr/bin/g++" >> $GITHUB_ENV
+
     - name: Setup VS Developer Command Prompt
       if: ${{ runner.os == 'Windows' }}
       uses: ilammy/msvc-dev-cmd@v1