Create diagnostic archive as option

Nobody has claimed this yet.

### What problem does your feature request solve?
Following #1326, creation of a final archive zip file now appears to happen by default. It would be preferable (essential?) if this is only created if user-specified option is enabled. i.e. for most users, generating zip file likely only increases total disk usage from CSET?

Change should also only create a zip file containing specified output filetypes (e.g. .png and .nc only??)

This change caught me out when running a cset bake test command, and found the task hanging as creating a zip file of * in a user-specified output directory which also contained the original data in large pp files).

### Describe the solution you'd like
Add a workflow and cset-bake options, which I suggest should be F by default like "--create-archive", that would trigger the final zip creation in cset bake.

### Describe alternatives you've considered
Think this issue is required to avoid increased length of time for some simple CSET applications. Apologies that I did not pick this up in original review to #1326

Start by locating the cset-bake option handling and the workflow that creates the final archive, then review the change referenced in #1326. Confirm how output filetypes are selected and whether archive creation is disabled by default. Done means an explicit archive option enables a zip containing only the specified output types, while ordinary cset-bake runs avoid creating it.

Written by the indexing model from the issue text.