ci: the Windows job gates (experimental: false) (P11c)

The port is in; the portable (windows-latest) job no longer hides behind
continue-on-error. Its first run is the first run of every Windows arm
(see docs/testing.md "P11c"), so the EM should watch that run before
merging.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

.github/workflows/check.yml

@@ -48,7 +48,7 @@ jobs:
           - os: macos-latest
             experimental: false
           - os: windows-latest
-            experimental: true
+            experimental: false
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}
     # Same disk discipline as above: no incremental caches, line tables only.