Add progress message to builtins_bzl_zip target

This is just a small quality of life improvement.

This is originally from PR #16806; it was split off to make submitting the
main change of that PR easier.

PiperOrigin-RevId: 500222350
Change-Id: I274f527340086a131d81f11a123bde240c87d34c

src/main/starlark/builtins_bzl/BUILD

@@ -21,6 +21,7 @@ genrule(
     cmd = "$(location //src:zip_builtins)" +
           " ''" +  # system zip
           " $@ src/main/starlark/builtins_bzl $(SRCS)",
+    message = "Building builtins_bzl.zip",
     output_to_bindir = 1,
     tools = ["//src:zip_builtins"],
     visibility = [