different cache file names needed for zstd with or without --long?

To be honest, I didn't try this, just something that came to my mind when having a short look at the sources.
So if this is not an issue actually, just close this.
But as far as the man page of zstd says, if you use --long with a value lager than 27 on compression, you also have to specify it on decompression.

But following the current logic, if you create a cache on a system with new zstd where --long is supported and then try to restore the cache on a system with old zstd where --long is not supported, the decompression will probably fail.
So I think a separate file name for zstd with --long is necessary.

No file or test is named. Start by tracing the toolkit's cache naming and zstd compression/decompression logic, then check the zstd man page and reproduce the described mismatch between --long and older zstd versions. Done means confirming whether the failure occurs and documenting or implementing the required cache-name separation if it does.