fix(ci): harden nightly status guards for ci setup tasks

Replace existence-based status checks with nightly-specific assertions
using `rustup run nightly rustc --version`. Prevents status guards from
passing when only stable Rust is pre-installed on GitHub-hosted runners.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pythoninthegrass

taskfiles/ci.yml

@@ -42,7 +42,7 @@ tasks:
       - test -x "{{.CARGO_BIN}}/rustup"
       - test -x "{{.CARGO_BIN}}/rustc"
       - test -x "{{.CARGO_BIN}}/cargo"
-      - rustc --version
+      - rustup run nightly rustc --version
 
   setup-rust-windows:
     desc: "Configure rustup nightly toolchain (Windows)"
@@ -53,8 +53,7 @@ tasks:
       - rustc --version
       - cargo --version
     status:
-      - rustup which rustc
-      - rustc --version
+      - rustup run nightly rustc --version
 
   setup-cargo-tools:
     desc: "Install cargo-binstall and tauri-cli"