add builder README.md and cleanup script

[harp-intel]

This pull request improves the documentation and logic for the PerfSpect build system, with a focus on clarifying and optimizing the caching of external tool binaries. The main changes include a new, detailed README for the build system, simplification and clarification of the build script's cache logic, and an update to the GitHub Actions workflow to ensure cache invalidation works as intended.

Documentation improvements:

• Added a comprehensive README.md to the builder/ directory, explaining the PerfSpect build process, caching mechanisms, and build artifacts, including detailed flow diagrams and environment variable explanations.

Build script enhancements:

• Refactored builder/build.sh to clarify the use of the tools binary cache, including renaming variables for clarity (e.g., USE_CACHE → USE_TOOLS_CACHE), and improving the logic that determines when to use cached binaries versus rebuilding from source.
• Updated the build script to select the appropriate Docker build command and cache options based on whether the build is running in GitHub Actions or locally, further simplifying the logic and making the cache usage more explicit.

CI workflow update:

• Changed the cache key in .github/workflows/build-test.yml to hash all files in the tools/ directory (tools/**), ensuring that any change to the tools triggers cache invalidation and a rebuild of the tools image.