gVisor bug has been hanging sandboxes since 2023

Since March 2023 gVisor's spinning queue was not shifted by the random memory-layout gap, so with a small gap it could land on a thread stack and hang the sandbox. The fix is one line in stub_unsafe.go; with it the repro ran clean 20 out of 20 times.

• Failure odds are about 1 in 114,000 at 16 CPUs and 1 in 35,000 at 64
• Affected systrap builds range from release-20230417.0 to release-20261005.0
• Until upgrading, --systrap-disable-fast-path avoids the spinning queue at a syscall-latency cost
• The fix landed in commit c7f458aa3, the first release after release-20261005.0