add env_var ZLUDA_CACHE_DIR override

[sfinktah]

This PR introduces the ZLUDA_CACHE_DIR environment variable, aligning ZLUDA with similar conventions used by other caching compilers, such as:

• TRITON_CACHE_DIR
• TORCHINDUCTOR_CACHE_DIR
• NUMBA_CACHE_DIR

Key Benefits:

• Enables users to define a custom cache directory for ZLUDA, allowing better separation of compiled artifacts.
• Facilitates cleaning experimental builds without re-triggering the recompilation of cached material, improving development workflows.
• Provides utility for advanced users who understand caching behavior and require more control over build processes.

This change is expected to be particularly useful for the small but technically savvy subset of users who manage cache directories for optimization and experimentation.