run: Always create a fontconfig cache dir in the app dir

Otherwise fontconfig falls back to a shared ~/.fontconfig dir
which means caches are not per-app, and is not necessarily accessible.

(cherry picked from commit fef8743f26)

Closes: #1115
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f