fix: persist OG image cache outside act runner workspace

The cache was written to .cache/og/ relative to the workspace, which is
under /usr/local/git/.cache/act/<unique-hash>/hostexecutor/ — a new path
per run, so every build regenerated all images from scratch.

OG_CACHE_DIR env var now controls the cache path (resolved to an absolute
path). CI sets it to /usr/local/git/.cache/og, which survives between runs.
Locally it still defaults to .cache/og inside the project dir.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/deploy.yml

@@ -125,6 +125,7 @@ jobs:
           GITEA_URL: https://gitea.giersig.eu
           GITEA_INTERNAL_URL: http://127.0.0.1:3000
           GITEA_ORG: giersig.eu
+          OG_CACHE_DIR: /usr/local/git/.cache/og
 
       - name: Deploy via rsync
         run: |