`nvptx-none-run.exp` uses a hard-coded path to its lock file

[tschwinge]

..., /tmp/nvptx-run, which may not be portable enough. On the other hand, the lock file really is meant to be system-wide. Perhaps use a lock backed by shm_open -- but that would have its own portability issues...

[tschwinge]

With commit b1e277c "Replace 'nvptx-none-run-single' with a locking mechanism in 'nvptx-none-run.exp'", locking has moved into nvptx-none-run.exp, and the (changed) "default /tmp/gpu.lock may be overridden by setting the NVPTX_NONE_EXECUTION_LOCK_FILE environment variable".