Replace dtolnay/rust-toolchain action with rustup show

[shaanmajid]

Replaces dtolnay/rust-toolchain@stable with rustup show, which installs the toolchain from rust-toolchain.toml directly. Follows up on #593 (comment).

• Eliminates the dtolnay/rust-toolchain third-party dependency
• Currently, the action installs latest stable (1.94.0), but cargo ignores it in favor of
  the 1.93.1 pinned in rust-toolchain.toml
• Adds CARGO_INCREMENTAL: 0 to disable incremental compilation in CI, where builds always
  start clean and the cached artifacts are never reused.
• Adds a CARGO_HOME fallback in the windows-11-arm nextest workaround

The last two compensate for side-effects of the removed action: dtolnay/rust-toolchain exported both CARGO_INCREMENTAL=0 and CARGO_HOME into GITHUB_ENV as part of its setup. CARGO_INCREMENTAL affects all runners; CARGO_HOME only affected windows-11-arm, where the runner image doesn't set it by default.

[deivid-rodriguez]

This is great, thank you!