Manifest: from eio_main to eio_posix

This is basically a revert of !18354 (merged) that was (wrongly?) merged assuming that the linux backend (using io uring) was working. It seems that it is not the case so we need to switch back to posix backend. Otherwise, I'm getting

octez-dal-node: internal error, uncaught exception:
                Unix.Unix_error(Unix.ENOMEM, "io_uring_queue_init", "")
                Raised at Stdlib__Domain.join in file "domain.ml", line 299, characters 16-24
                Called from Eio_linux.Domain_mgr.run.(fun) in file "lib_eio_linux/eio_linux.ml", line 291, characters 21-55
                Called from Eio__core__Trace.with_span in file "lib_eio/core/trace.ml", line 55, characters 8-13
                Re-raised at Eio__core__Trace.with_span in file "lib_eio/core/trace.ml", line 60, characters 4-39
                Called from Eio__core__Fiber.fork_daemon.(fun) in file "lib_eio/core/fiber.ml", line 34, characters 10-14
                Re-raised at Eio__core__Switch.maybe_raise_exs in file "lib_eio/core/switch.ml", line 119, characters 21-56
                Called from Eio__core__Switch.run_internal in file "lib_eio/core/switch.ml", line 149, characters 4-21
                Called from Eio__core__Cancel.with_cc in file "lib_eio/core/cancel.ml", line 118, characters 8-12
                Re-raised at Eio__core__Cancel.with_cc in file "lib_eio/core/cancel.ml", line 120, characters 50-58
                Called from Stdlib__Fun.protect in file "fun.ml", line 34, characters 8-15
                Re-raised at Stdlib__Fun.protect in file "fun.ml", line 39, characters 6-52
                Called from Eio__core__Switch.run_internal in file "lib_eio/core/switch.ml", line 133, characters 8-12
                Re-raised at Eio__core__Switch.maybe_raise_exs in file "lib_eio/core/switch.ml", line 119, characters 21-56
                Called from Eio__core__Switch.run_internal in file "lib_eio/core/switch.ml", line 149, characters 4-21
                Called from Eio__core__Cancel.with_cc in file "lib_eio/core/cancel.ml", line 118, characters 8-12
                Re-raised at Eio__core__Cancel.with_cc in file "lib_eio/core/cancel.ml", line 120, characters 50-58
                Called from Eio_unix__Thread_pool.run in file "lib_eio/unix/thread_pool.ml", line 108, characters 8-13
                Re-raised at Eio_unix__Thread_pool.run in file "lib_eio/unix/thread_pool.ml", line 113, characters 4-39
                Called from Eio_linux__Sched.run.(fun) in file "lib_eio_linux/sched.ml", line 468, characters 22-90
                Re-raised at Eio_linux__Sched.run in file "lib_eio_linux/sched.ml", line 479, characters 22-57
                Called from Eio_linux__Sched.with_eventfd in file "lib_eio_linux/sched.ml", line 506, characters 8-18
                Re-raised at Eio_linux__Sched.with_eventfd in file "lib_eio_linux/sched.ml", line 511, characters 4-39
                Called from Eio_linux__Sched.with_sched in file "lib_eio_linux/sched.ml", lines 543-545, characters 8-109
                Re-raised at Eio_linux__Sched.with_sched in file "lib_eio_linux/sched.ml", line 556, characters 8-43
                Called from Cmdliner_term.app.(fun) in file "cmdliner_term.ml", line 24, characters 19-24
                Called from Cmdliner_term.ret.(fun) in file "cmdliner_term.ml", line 41, characters 25-32
                Called from Cmdliner_eval.run_parser in file "cmdliner_eval.ml", line 35, characters 37-44

This is a prerequisite to running the DAL amplifier into a Eio domain.

This might be reverted avec lib_bees fix -- to come.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR