[CB] Override number of Spyre blocks: replace env var with top level argument

[yannicks1]

[CB] Override number of Spyre blocks: replace env var with top level argument

depreciating the env var VLLM_SPYRE_N_BLOCKS and use the top level argument num_gpu_blocks_override instead to override the number of avail spyre kv cache blocks.

[github-actions]

👋 Hi! Thank you for contributing to vLLM support on Spyre.
Just a reminder: Make sure that your code passes all the linting checks, otherwise your PR won't be able to be merged. To do so, first install the linting requirements, then run format.sh and commit the changes. This can be done with uv directly:

uv sync --frozen --group lint --active --inexact

Or this can be done with pip:

uv pip compile --group lint > requirements-lint.txt
pip install -r requirements-lint.txt
bash format.sh

Now you are good to go 🚀

[maxdebayser]

LGTM