Integrating cmake project into bazel monorepo

Question

I'm playing around with bazel (rather new to it) and want to integrate one of my projects that needs to use cmake due to the use of JUCE.

I built a minimal example but I'm getting the error: BUILD.bazel:5:6: output 'cmake_test/projectA/helloworldexec' was not created

my folder structure is

cmake_test
  projectA
    src
      helloworld.cpp
    BUILD.bazel
    CMakeLists.txt
  BUILD.bazel
WORKSPACE.bazel

WORKSPACE.bazel basically just sets up rules_foreign_cc

cmake_test/BUILD:

load("@bazel_skylib//rules:build_test.bzl", "build_test")
load("@rules_cc//cc:defs.bzl", "cc_test")
load("@rules_foreign_cc//foreign_cc:defs.bzl", "cmake")

cmake(
    name = "projectA",
    generate_args = ["-GNinja"],
    lib_source = "//cmake_test/projectA:a_srcs",
    out_binaries = ["helloworldexec"],
    out_bin_dir = ""
)

cmake_test/projectA/BUILD

load("@rules_cc//cc:defs.bzl", "cc_library")

filegroup(
    name = "a_srcs",
    srcs = glob(["**"]),
    visibility = ["//visibility:public"],
)

CMakeLists.txt

cmake_minimum_required(VERSION 2.23.0)

project(helloworld)

add_executable(helloworldexec src/helloworld.cpp)

helloworld.cpp is just a main function that couts hello world.

Why isn't this built?