[infra] Add --force flag to git submodule update in script.

[ScottTodd]

Context: #19538 (comment)

One workflow failed on a self-hosted runner after changing a submodule pin:

+ git submodule update --init --jobs 8 --depth 1 third_party/benchmark third_party/cpuinfo third_party/flatcc third_party/googletest third_party/hip-build-deps third_party/hsa-runtime-headers third_party/musl third_party/spirv_cross third_party/tracy third_party/vulkan_headers third_party/webgpu-headers
From https://github.com/google/benchmark
 + c19cfee...f4f93b5 main       -> origin/main  (forced update)
From https://github.com/google/benchmark
 * branch            99bdb2127d1fa1cff444bbefb814e105c7d20c45 -> FETCH_HEAD
error: Your local changes to the following files would be overwritten by checkout:
	.github/workflows/pre-commit.yml
	.github/workflows/test_bindings.yml
       ...

Please commit your changes or stash them before you switch branches.
Aborting
fatal: Unable to checkout '99bdb2127d1fa1cff444bbefb814e105c7d20c45' in submodule path 'third_party/benchmark'

This adds the --force flag to try proceeding anyways. Anyone calling this script is already delegating the submodule update behavior to it.