Fix racy write of temporary files while staging virtual inputs for the sandbox.

When staging virtual inputs without delayed materialization, `SandboxHelpers`
performs atomic writes by first staging virtual inputs into a temporary file
and later moving it to the target destination. This is achieved by performing
the initial writes into a file with a uniquifying suffix. This suffix happens
to currently always be hardcoded to `.sandbox`, which means that staging the
same virtual inputs for 2 actions may race on that.

Fix the race condition by providing a truly unique suffix for the temporary
file.

PiperOrigin-RevId: 351613739