feat: add caller cache support to reusable build and test workflows

[vyasr]

Summary

Adds an opt-in, caller-configured cache interface to the reusable Conda and wheel build/test workflows. It replaces workflow-specific cache inputs with a common mechanism that supports multiple workspace-relative paths.

Callers provide:

• cache-paths: newline-separated files or directories for actions/cache;
• cache-key-prefix: cache namespace;
• cache-key-files (optional): source or dependency globs hashed into the exact key, with a matrix-compatible restore prefix;
• cache-environment (optional): newline-separated NAME=VALUE variables for tools that must use a workspace-mounted cache directory.

Every cache key is scoped to the compatible workflow matrix. Test keys include GPU, driver, and dependency variants; build keys use the build matrix dimensions.

This supersedes #629's Numba-specific cache input, which has been closed. Numba can use this generic interface with .cache/numba, Python/dependency key files, and NUMBA_CACHE_DIR.

Validation

• YAML and workflow-lint validation will be run after the remaining wheel-build wiring is complete.