Make buck_flavors_deterministic_test more robust

Summary:
On centos8 devservers, this test failed on bizarre buck-out/tmp java.nio.file.NoSuch
FileException. I can't tell exactly what going on with rm -rf buck-out, but my guess would be that it puts the running buckd in a bad state.

using 'buck clean' rather than 'rm -rf buck-out' makes buck happier

Reviewed By: jvillard

Differential Revision: D25534471

fbshipit-source-id: 215f993e3

infer/tests/build_systems/buck_flavors_deterministic/Makefile

@@ -20,12 +20,12 @@ $(OBJECTS): $(JAVA_SOURCE_FILES)
 	$(BUCK) build --no-cache $(BUCK_TARGET))
 
 differences.exp.test: $(CLANG_DEPS) $(SOURCES) $(MAKEFILE_LIST)
-	$(QUIET)$(REMOVE_DIR) buck-out && \
+	$(QUIET)$(BUCK) clean && \
 	$(call silent_on_success,Running Buck flavors capture a first time,\
 	$(INFER_BIN) $(INFER_OPTIONS) capture --buck-clang --results-dir $(CURDIR)/infer-out -- \
 	  $(BUCK) build --no-cache $(BUCK_TARGET) && \
 	find buck-out/gen/src/infer-out-* -type f | xargs cat | $(SHASUM) > capture_hash-1.sha)
-	$(QUIET)$(REMOVE_DIR) buck-out && \
+	$(QUIET)$(BUCK) clean && \
 	$(call silent_on_success,Running Buck flavors capture a second time,\
 	$(INFER_BIN) $(INFER_OPTIONS) capture --buck-clang --results-dir $(CURDIR)/infer-out -- \
 	  $(BUCK) build --no-cache $(BUCK_TARGET) && \