Doc: Document WORKSHOP_CACHE for local setup

[shizuku198411]

Description

Document WORKSHOP_CACHE in the local daemon setup instructions.

Following the current instructions with only WORKSHOP set can make workshopd run --create-dirs try to create /var/cache/workshop, which fails for a regular user with:

cannot run workshop: mkdir /var/cache/workshop: permission denied

This PR adds WORKSHOP_CACHE to the local development environment setup so the daemon uses a user-writable cache directory.

Fixes #791

Self-review quick check

• Make decisions that cost a lot to reverse explicit in the PR description.
• Avoid nested conditions.
• Delete dead code and redundant comments.
• Normalise symmetries by sticking to doing identical things identically.

// one way to handle errors
if err := f(); err != nil {
   ...
}

// one way to handle multiple returns
val, err := f()
if err != nil {
   ...
}
...

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Docs

Procedure:

• I have checked and added or updated relevant documentation.
• I have checked and added or updated relevant release notes.
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Content:

• Headings and titles accurately describe the content.
• New and updated pages include correct metadata.
• Documentation tests are added or updated where applicable (for tutorial/ and how-to/ sections).
• Documentation follows the style guide.
• If needed, docs/.coverage.yaml updated, coverage tags added (.. artefact).

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Or:

• I confirm the PR has no implications for documentation.