fix(ci): fix YAML syntax error in build.yaml that silently disabled Actions

The unindented heredoc terminator (<<EOF2 ... EOF2 flush-left) broke the
YAML block-scalar indentation contract for the `run: |` step, causing
Forgejo to log "ignore invalid workflow build.yaml: yaml: line 75:
could not find expected ':'" and silently skip the workflow entirely
on every push since this line was introduced. No builds have run on
main or high-performance since 2026-07-05 as a result. Replaced the
multi-line heredoc with a single-line bash here-string, which needs
no extra indentation and keeps the shell logic identical.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/build.yaml

@@ -71,9 +71,7 @@ jobs:
               apps/ugc_content_creators/*) echo ugc-content-creators >> /tmp/changed-services.txt ;;
               apps/cron/*) echo cron >> /tmp/changed-services.txt ;;
             esac
-          done <<EOF2
-$CHANGED_FILES
-EOF2
+          done <<<"$CHANGED_FILES"
 
           sort -u /tmp/changed-services.txt -o /tmp/changed-services.txt