Set CC as well for pkgbuild:::has_compiler_colored_diagnostics()

Otherwise this will incorrectly return `TRUE` because it compiles a C program rather than a C++ one to check if color support is available, which adds the `-fdiagnostics-color=always` flag, but g++ 4.8 doesn't actually support that, so the compilation of the C++ code in the vignettes would fail

Author: DavisVaughan

.github/workflows/R-CMD-check.yaml

@@ -69,7 +69,7 @@ jobs:
           sudo apt update
           sudo apt-get install -y g++-4.8
           mkdir ~/.R/
-          echo $'CXX1X=g++-4.8\nCXX11=g++-4.8' >> ~/.R/Makevars
+          echo $'CXX1X=g++-4.8\nCXX11=g++-4.8\nCC=gcc-4.8' >> ~/.R/Makevars
 
       - name: Install macOS system dependencies
         if: runner.os == 'macOS'