feat(dd): use wasi_ext feature on nightly

[xtqqczze]

Enable the unstablewasi-ext feature for nightly WASI builds.

• Detect nightly Rust in build.rs.
• Set the nightly cfg flag.
• Gate wasi_ext behind nightly and target_os = "wasi".

Context: #12258.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/df/df-output is no longer failing!

[sylvestre]

still not a fan of this
esp without any comment on the why

[xtqqczze]

added a couple of comments

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.20 MB (+1016 KB, +0.66%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[codspeed]

Merging this PR will not alter performance

✅ 377 untouched benchmarks
⏩ 54 skipped benchmarks¹

---

_{Comparing xtqqczze:nightly-wasi (90c113e) with main (ea21b10)}

Footnotes

1. 54 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[xtqqczze]

@sylvestre I'm minded to close this, and convert to an issue blocked on rust-lang/rust#71213.

[eduardomourar]

As this feature in standard library Rust is unlikely to become stable soon, I suggest rewriting this PR to use rustix::fs::fstat/FileType plus std::os::fd instead of #![feature(wasi_ext)], dropping the build.rs/nightly-detection machinery entirely.

[xtqqczze]

Tracked by #15215.

[oech3]

Personally, RUSTC_BOOTSTRAP=1 on the wasi CI is acceptible for me.