Enable the cachewebsite build option by default, including on our CI

Originally created by @intrigeri on #17439 (Redmine)

Enabling the new cachewebsite build option (#15342 (closed)) by default should lower by 25% the total runtime of a Tails images build job on our CI.
It would take up to 5 extra GiB on isobuilders’ /var/lib/libvirt/images.
Note that this option is automatically disabled when building a release (i.e. from a tag), to be on the safe side.

Here’s the plan:

1. Ask our developers to use the cachewebsite option locally, in order to iron out whatever bugs are left in the implementation (I think I’ll do that anyway) done (2020-02-04)
2. Ensure our isobuilders have enough space to store the cache done
3. While waiting for feedback from developers, enable this on my local Jenkins for a few weeks, in order to learn about potential Jenkins-specific problems this may cause
4. On a topic branch, enable cachewebsite by default and add a cachewebsite=no option to allow disabling this feature

Feature Branch: feature/17439-enable-cachewebsite-by-default

Parent Task: sysadmin#16960

Related issues

• Blocked by #15342 (closed)
• Blocked by #17481 (closed)