internal: Fix caches being orphaned from their directory. After clearing caches, the instances in memory don't have a file-system directory anymore. This means that either caching data would crash the app, or, in the case of baking a site, would make the workers create the directory but collide randomly while doing so at the same time.

---
in:
    pages/foo.md: |
        Foo!
    pages/bar.md: |
        Bar!
    pages/allpages.md: |
        {% for p in site.pages -%}
        {{p.url}}
        {% endfor %}
outfiles:
    allpages.html: |
        /
        /allpages.html
        /bar.html
        /foo.html