fix(ci): use fully qualified MSVC toolchain name on Windows

RUSTUP_TOOLCHAIN=nightly-2026-02-09 is ambiguous — rustup resolves it
using the default host triple, which gets reset to GNU by the
setup-rust-toolchain action's rustup-init reading a stale system-level
settings file. Use the fully qualified name with -x86_64-pc-windows-msvc
suffix on Windows to eliminate the ambiguity.

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

Author: pythoninthegrass

taskfiles/ci.yml

@@ -21,7 +21,7 @@ vars:
 env:
   PATH: "{{.CARGO_BIN}}:{{.MISE_SHIMS}}:{{.PATH}}"
   RUSTC_WRAPPER: ""
-  RUSTUP_TOOLCHAIN: "{{.PINNED_RUST}}"
+  RUSTUP_TOOLCHAIN: '{{if eq OS "windows"}}{{.PINNED_RUST}}-x86_64-pc-windows-msvc{{else}}{{.PINNED_RUST}}{{end}}'
 
 tasks:
   setup: