possible threading issue in the `io_uring_context`

[ericniebler]

@lucteo reports in #1360 that he encountered the following test failure after running the stdexec tests 1000 times.

654: Test command: /github/workspace/build/gcc11-release/test/test.stdexec "io_uring_context schedule_after -1s"
654: Working Directory: /github/workspace/build/gcc11-release/test
654: Test timeout computed to be: 60
654: Filters: io_uring_context schedule_after -1s
654:
654: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
654: test.stdexec is a Catch v2.13.6 host application.
654: Run with -? for options
654:
654: -------------------------------------------------------------------------------
654: io_uring_context schedule_after -1s
654: -------------------------------------------------------------------------------
654: /github/workspace/test/exec/test_io_uring_context.cpp:314
654: ...............................................................................
654:
654: /github/workspace/test/exec/test_io_uring_context.cpp:337: FAILED:
654:   CHECK( is_called_1 == true )
654: with expansion:
654:   false == true
654:
654: ===============================================================================
654: test cases: 1 | 1 failed
654: assertions: 3 | 2 passed | 1 failed
654:
654/704 Test #654: io_uring_context schedule_after -1s ...........................................................................................***Failed    0.00 sec
Filters: io_uring_context schedule_after -1s