ci(build): flip windows lane to self-hosted VM217 (fork-gated)

VM217 (c2pool-windows-217) toolchain probe run 28679257030 is green across
Python 3.12, Conan 2, CMake, Git and MSVC/VS 2022 C++ build tools. Route
internal PRs/pushes to the self-hosted runner via the #439 fork-gate expr;
fork PRs keep GitHub-hosted windows-2022. setup-python gated to github-hosted
so VM217 uses its pre-provisioned interpreter. Last GitHub-hosted C++ lane.

Author: frstrtr

.github/workflows/build.yml

@@ -37,7 +37,9 @@ jobs:
           sudo apt-get install -y --no-install-recommends \
             g++ ccache cmake make libleveldb-dev libsecp256k1-dev
 
+      # Python is pre-provisioned on VM217; only provision on GitHub-hosted forks.
       - uses: actions/setup-python@v6
+        if: ${{ runner.environment == 'github-hosted' }}
         with: { python-version: '3.12' }
 
       - name: Install Conan 2
@@ -721,14 +723,12 @@ jobs:
     name: Windows x86_64
     # dash-slice campaign: skip Windows for PRs carrying the dash-linux-only label (re-enabled at block-viable gate, ~07-01)
     if: ${{ !(github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'dash-linux-only')) }}
-    # GitHub-hosted windows-2022 (ships cmake + MSVC/VS 2022 + the SDK).
-    # VM217 (c2pool-windows-217) is still a bare runner: the self-hosted flip
-    # failed at actions/setup-python@v6 (cannot provision Python on VM217) and
-    # no VS 2022 Build Tools are installed yet. FOLLOW-UP: once vm-fleet installs
-    # VS 2022 Build Tools (C++ workload) + cmake + python on VM217 and it builds
-    # c2pool clean, a follow-up PR flips this to the fork-gated self-hosted expr.
-    # web-static + pplns self-hosted routes on this branch are proven green.
-    runs-on: windows-2022
+    # Internal PRs / pushes -> self-hosted c2pool-windows-217 (VM217): toolchain
+    # probe run 28679257030 green (Python 3.12, Conan 2, CMake, Git, MSVC/VS 2022
+    # C++ build tools all provisioned). Fork PRs fall back to GitHub-hosted
+    # windows-2022 (ships cmake + MSVC/VS 2022 + SDK). Completes the #439
+    # self-hosted migration -- Windows was the last GitHub-hosted C++ lane.
+    runs-on: ${{ (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository) && fromJSON('["self-hosted","Windows","X64","c2pool-build"]') || 'windows-2022' }}
     steps:
       - uses: actions/checkout@v6