build: silencing absl logspam (envoyproxy#21048)

The absl flagsaver logs every time it restores flags and this is the best way I can find to silence it
https://github.com/abseil/abseil-cpp/blob/master/absl/base/internal/raw_logging.cc#L147

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

.bazelrc

@@ -19,6 +19,8 @@ build --host_force_python=PY3
 build --java_runtime_version=remotejdk_11
 build --tool_java_runtime_version=remotejdk_11
 build --platform_mappings=bazel/platform_mappings
+# silence absl logspam.
+build --copt=-DABSL_MIN_LOG_LEVEL=4
 
 # Pass PATH, CC, CXX and LLVM_CONFIG variables from the environment.
 build --action_env=CC --host_action_env=CC