filesystem: ErrorCode::IsDirectory when opening directory w/o READ

[wingo]

On Windows, it was possible to return a directory descriptor if READ wasn't in the permissions. Fixes wasmtime for
WebAssembly/wasi-testsuite#176.

[wingo]

In all honesty I haven't tested on Windows, it's just that this patch should fix the issue. To repro, check out the prod/testsuite-base branch of https://github.com/WebAssembly/wasi-testsuite and run:

wasmtime -Wcomponent-model-async -Sp3,http --dir 'tests\rust\testsuite\wasm32-wasip3\fs-tests.dir::fs-tests.dir' 'tests\rust\testsuite\wasm32-wasip3\filesystem-flags-and-type.wasm'

[wingo]

Rebased to fix the rustfmt error

[wingo]

My goodness, there are many CI checks in this project :) Patch fixes unused-variable warning/error when compiling for Windows.

[rvolosatovs]

My goodness, there are many CI checks in this project :) Patch fixes unused-variable warning/error when compiling for Windows.

You can add prtest:full to commit message description to trigger a full CI run for cases like these

[alexcrichton]

Test-wise this is something that'll be caught when updating to the latest wasi-testsuite, right? If so, I think it's ok to skip a test here. Mind leaving a comment though on this match arm explaining that this is sync'ing windows semantics to unix semantics?