ci: pin check-minver to nightly-2026-04-25 (#905)

Newer nightlies reject the `rustc_*` attributes used in `rustix
v0.36.4`, which `cargo minimal-versions` resolves to. The `check-minver`
job has `continue-on-error: true` so it isn't blocking, but it's been
red on every PR for the past week or so. Pin until a newer nightly is
known-good.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: max-sixty

.github/workflows/tests.yml

@@ -104,7 +104,10 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly
+          # Pinned: newer nightlies reject the `rustc_*` attributes used in
+          # `rustix v0.36.4`, which `cargo minimal-versions` resolves to.
+          # Bump once a newer nightly is known-good.
+          toolchain: nightly-2026-04-25
       - uses: Swatinem/rust-cache@v2
       - uses: taiki-e/install-action@cargo-hack
       - uses: taiki-e/install-action@cargo-minimal-versions