Change cache default path

[forsyth2]

Request criteria

• I searched the zstash GitHub Discussions to find a similar question and didn't find it.
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Issue description

From @TonyB9000 at #341 (comment): Where "--cache" is specified, but NOT a fully-qualified path, I think the default behavior should be to place it in the user's current directory. By placing it in the tail of the source directory, it could interfere with some processing that is designed to "process all contents of a directory", not expecting a new file or directory to have appeared.