Fix CI badge cache purge failing when no camo URLs found

With pipefail enabled, grep exiting 1 on no matches fails the step.
Add xargs -r and || true so the purge is best-effort only.

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

.forgejo/workflows/ci.yml

@@ -21,4 +21,4 @@ jobs:
           curl -sL https://github.com/opennomad/systab \
             | grep -oP 'https://camo\.githubusercontent\.com/[^"]+' \
             | sort -u \
-            | xargs -I{} curl -sX PURGE '{}'
+            | xargs -r -I{} curl -sX PURGE '{}' || true