perf: optimize OpenCV static build caching for QEMU docker builds

[Vladush]

Description

This PR fixes an issue where the OpenCV static library was being built from scratch on every run for riscv64 and i386 QEMU Docker builds. Because the debian:sid base image digest updates frequently, GitHub Docker registry layer caching was constantly invalidated.

Changes introduced:

• Added actions/cache steps to the i386 and riscv64 CI jobs to cache opencv_static directly on the GitHub host runner.
• Updated docker/Dockerfile.i386 and docker/Dockerfile.riscv64 to seamlessly move the cached opencv_static folder during the build process, bypassing the ~2 hours cross-compilation step.
• Included an rm -rf ./opencv_static_cached pre-extraction step to ensure robust caching behavior without nested directory artifacts across cache hits.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement (CI)

How Has This Been Tested?

• Simulated Docker layer caching miss to verify the script properly checks for the /app/opencv_static/lib/libopencv_core.a file.
• Validated Dockerfile COPY commands correctly pick up the cached artifacts without breaking local non-CI docker builds (due to .dockerignore settings remaining intact).

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes