fix: always set directory open flag in open_ambient_dir

[rvolosatovs]

Note, that this is the only way to actually acquire a directory from AmbientAuthority when running in Wasmtime.
Fix for that is in-progress at bytecodealliance/wasmtime#4947

All this should do is fail if the FD is not a directory. Currently Wasmtime Snapshot 1 implementation incorrectly uses this bit value to determine whether to open the FD in "file mode" or "directory mode". Setting this bit is a workaround for that issue, but it's also just generally a good thing to do.

[rvolosatovs]

CI failures seem unrelated:

 [cap-primitives 1.0.0-rc1] For more information about this error, try `rustc --explain E0554`.
error[E0308]: arguments to this function are incorrect
    --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/rustix-0.36.0-rc1/src/backend/libc/fs/makedev.rs:15:14
     |
15   |     unsafe { c::makedev(maj as i32, min as i32) }
     |              ^^^^^^^^^^ ----------  ---------- expected `u32`, found `i32`
     |                         |
     |                         expected `u32`, found `i32`
     |
note: function defined here
    --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.133/src/unix/linux_like/android/mod.rs:2794:20
     |
2794 |     pub {const} fn makedev(ma: ::c_uint, mi: ::c_uint) -> ::dev_t {
     |                    ^^^^^^^
help: you can convert an `i32` to a `u32` and panic if the converted value doesn't fit
     |
15   |     unsafe { c::makedev((maj as i32).try_into().unwrap(), min as i32) }
     |                         +          +++++++++++++++++++++
help: you can convert an `i32` to a `u32` and panic if the converted value doesn't fit
     |
15   |     unsafe { c::makedev(maj as i32, (min as i32).try_into().unwrap()) }
     |                                     +          +++++++++++++++++++++

[sunfishcode]

The makedev error in CI is indeed unrelated, and it's now fixed on main. Would you mind rebasing this?

[rvolosatovs]

@sunfishcode updated, PTAL

[sunfishcode]

Thanks!