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>
diff --git a/infra/cmake/build_skia.sh b/infra/cmake/build_skia.sh
index dbd50dc..2d36d9e 100755
--- a/infra/cmake/build_skia.sh
+++ b/infra/cmake/build_skia.sh
@@ -27,6 +27,8 @@
 gn gen out/CMAKE --args='is_debug=false' --ide=json --json-ide-script=../../gn/gn_to_cmake.py
 
 cd $SKIA_DIR/out/CMAKE
+export CC=/usr/local/bin/clang
+export CXX=/usr/local/bin/clang++
 cmake -G"CodeBlocks - Unix Makefiles" .
 cmake --build . --parallel 8