Kokoro: Don't use verbose logging for make

When the build fails, it's very hard to find the compiler error in the spam.

Change-Id: Ic975f0a075e4104db82a58ef03e1ba643e520fe2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26031
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

Author: ben-clayton

kokoro/gcp_ubuntu/continuous.sh

@@ -12,7 +12,7 @@ git submodule update --init
 mkdir -p build && cd build
 
 cmake ..
-make --jobs=$(nproc) VERBOSE=1
+make --jobs=$(nproc)
 
 # Run the GLES unit tests.
 ./unittests