fix(caches/in-memory): allow explicit in-memory caches

[hsjobeki]

This fixes the verbose warnings of: #26747

Some users (such as me) don't have access to a writeable file system.
In those cases it would be nice to have a global flag to run deno with in-memory caches only.

The code for falling back to in-memory caches is already there but no way to use it without beeing spammed with concerning error messages.

I could need some help here because i didn't figure out how to properly register the cli flag.

[CLAassistant]

All committers have signed the CLA.

[dsherret]

What about just not logging when the raw error kind is a readonly file system? That would mean people wouldn't have to discover this flag to supress the warnings.

By the way, I'm not sure about a CLI flag for this. Maybe it should be an env var so all sub commands share the behaviour.

[dsherret]

Discussed internally and what would be preferable for this one is to first land a PR with this not warning for readonly file systems. After that's landed and if there's still a need, then have this behaviour triggered by an environment variable.