loop: install hooks relative to repo root, not working dir

An evergreen git bug.

Fixes missing hooks when started from a subdirectory.
diff --git a/loop/agent.go b/loop/agent.go
index 273964f..0a4555b 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -894,7 +894,7 @@
 			return fmt.Errorf("resolveRef: %w", err)
 		}
 
-		if err := setupGitHooks(a.workingDir); err != nil {
+		if err := setupGitHooks(a.repoRoot); err != nil {
 			slog.WarnContext(ctx, "failed to set up git hooks", "err", err)
 		}