Add CACHEDIR.TAG to .polywrap/ directory

[dOrgJelli]

Describe the bug The .polywrap/ directory used by the CLI does not properly represent itself as a "cache directory". To do this using an existing internet custom (used by projects like cargo) is to include a caching directory tag like so: CACHEDIR.TAG

Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by cargo.
# For information about cache directory tags see https://bford.info/cachedir/

Please visit the website listed to learn about how this tag is intended to be used.