Use Clang for CMake build

Despite the name, Build-Debian9-Clang-x86_64-Release-CMake was using
the default compiler, which is GCC, not Clang. Specify clang explicitly
in env vars for CMake.

Change-Id: Ibfad7b4de555a8a8acdddb8416582764b5a0640c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254639
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Ben Wagner aka dogben <benjaminwagner@google.com>
1 file changed