Gitian includes two caches where build-assets may be saved for use with
subsequent builds. These caches are only enabled if the "enable_cache" key
is enabled in the build descriptor.

Common cache:
All descriptors share this cache. It can be useful for storing fetched sources,
sharing build assets between descriptors, etc.

Per-descriptor cache
Files installed to this cache can only be seen by this descriptor. Use this to
store assets created as part of the build process, to avoid having to rebuild
them in future builds.

If caching is enabled, GBUILD_CACHE_ENABLED will be set to 1 in the build
script. In addition, GBUILD_PACKAGE_CACHE and GBUILD_COMMON_CACHE will be set
to their respective paths. GBUILD_PACKAGE_CACHE contains the descriptor's
"name" key as a means of separating caches. To add or update the cached files,
copy them to  these paths.

Before each build, all files and folders in the cache directories will be
transferred to the VM. After each successful build, the caches will be
transferred back to the builder (overwriting existing files as necessary).

It is entirely up to the user and the descriptors to verify that cached files
are actually suitable for re-use. Overwriting existing cached files is _highly_
discouraged, as that would likely lead to non-deterministic results. Unique
filenames should always be used.