vulkan: Set limit for task concurrency

[luciferous]

A common default for the maximum number of open files is 256, which can lead to asyncio.gather(*tasks) failing with Too many open files.

$ python ggml_vk_generate_shaders.py --glslc=$ANDROID_NDK_PATH/shader-tools/darwin-x86_64/glslc
ggml_vulkan: Generating and compiling shaders to SPIR-V
Traceback (most recent call last):
  File "/Users/neuman/Code.noindex/github/llama.cpp/ggml_vk_generate_shaders.py", line 2326, in <module>
    asyncio.run(main())
  File "/Users/neuman/Code.noindex/miniforge3/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/Users/neuman/Code.noindex/miniforge3/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "/Users/neuman/Code.noindex/github/llama.cpp/ggml_vk_generate_shaders.py", line 2294, in main
    await asyncio.gather(*tasks)
[...snip...]
OSError: [Errno 24] Too many open files

This change sets a reasonable concurrency limit for tasks (and therefore open files), without significant impact on run time.

[0cc4m]

Looks good, thank you!