[bugfix]: fix bash syntax error in modal ci pr checkout

The multi-line retry checkout command ended with a newline, so splicing it
into '{checkout_command} &&' left a lone '&&' on the line after 'done' — a
bash syntax error failing every PR-triggered shard before checkout. End the
f-string at 'done' so the composed script reads 'done &&'; verified all four
compositions (PR/direct x kernel/no-kernel) with bash -n.

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

fastvideo/tests/modal/pr_test.py

@@ -121,8 +121,7 @@ def run_test_command(test_command: str,
                 exit "$status"
             fi
             sleep $((attempt * 5))
-        done
-        """
+        done"""
         print(f"Using PR ref for checkout: {checkout_command}")
     else:
         checkout_command = f"git checkout {git_commit}"