Run actual build_gen bazel target in CI

[alexander-fenster]

Would be nice to run the actual java_binary target in CI to make sure we don't use Java features not supported by the lowest version we claim to support (1.8). Would prevent problems like in #145 where String.isBlank started failing for some users.

rules_gapic/bazel/BUILD.bazel

Lines 8 to 19 in 34c2750

	java_binary(
	name = "build_file_generator",
	srcs = glob(["src/main/java/**/*.java"]),
	resources = glob(["src/main/java/**/*.mustache"]),
	data = [":buildozer_binary"],
	args = ["--buildozer=$(location :buildozer_binary)"],
	create_executable = True,
	javacopts = ["-source", "1.8", "-target", "1.8"],
	jvm_flags = ["-Xmx1024m"],
	main_class = "com.google.api.codegen.bazel.BuildFileGenerator",
	visibility = ["//visibility:public"],
	)