Download should stick to zip for better cacheability

[calind]

Currently if you do a wp download --version=4.9.4 it will download and cache a .tar.gz file and if you afterward do a wp download --version=4.9.4 --skip-content for example it will download a new .zip archive.

I think it would be better if we can always stick to downloading zip files so you can reuse archives and save some bandwith, or be able to work disconnected.

[danielbachhuber]

There's some historical reason why both .tar.gz and .zip are used but I don't recall it off the top of my head. It'd be worthwhile to track the history down before considering any changes.

Here's some git blame you can use as a starting point: https://github.com/wp-cli/wp-cli/blame/c8ebfdbc53c2b58069b5cee902b0c7b4e1d2c634/php/commands/core.php#L117

Also, wp-cli/wp-cli#1532 was a really nasty bug that makes me hesitant to make significant changes to wp core download. I think the test coverage is good, but not great.