Avoid database locking issues for parallel Conan builds

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.

Start in python/aswfdocker/builder.py, especially the build() method, and trace how Conan caches are selected for each VFX version. Verify the change with parallel --version 2024 --version 2025 --version 2026 builds: package builds for one version remain serialized while different years use separate caches and can proceed concurrently.

Written by the indexing model from the issue text.