Doc: Document WORKSHOP_CACHE for local daemon setup

[shizuku198411]

Following the local daemon setup instructions in docs/contributing.rst causes workshopd to try to create /var/cache/workshop unless WORKSHOP_CACHE is also set.

As a regular user, this fails with:

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

Relevant code

The contributing guide currently documents WORKSHOP, but the daemon cache directory is configured separately through WORKSHOP_CACHE.

From internal/dirs/dirs.go:

workshopdDir = os.Getenv("WORKSHOP")
if workshopdDir == "" {
    workshopdDir = defaultBaseDir
}

cacheDir = os.Getenv("WORKSHOP_CACHE")
if cacheDir == "" {
    cacheDir = defaultCacheDir
}