Avoid database locking issues for parallel Conan builds

[jfpanisset]

Conan does not yet fully support parallel builds as of 2.18.1, which is reflected in the build() method in python/aswfdocker/builder.py, but SQLite database locking issues can be avoided by keeping separate caches. Different VFX years create a clear separation between different build contexts, and we don't want to reuse builds between VFX years anyway, so keeping separate per-year Conan caches could allow aswfdocker to run on multiple --version 2024 --version 2025 --version 2026 at once, serializing the package builds for a specific version, but parallelizing them across versions / VFX platform years.