ci(build): revert Windows job to GitHub-hosted windows-2022

VM217 (c2pool-windows-217) is a bare self-hosted runner with no build
toolchain — no cmake, no MSVC/VS 2022, no SDK — so the Windows lane fails
at configure. GitHub-hosted windows-2022 ships all of it.

Keep Linux (8 lanes -> VM905) and macOS (M4/Intel) on the self-hosted
fleet with the fork-gate; only the Windows job falls back to
github-hosted. Follow-up PR will flip Windows back to the fork-gated
self-hosted expr once VM217 has the VS 2022 Build Tools C++ workload +
cmake + deps installed and builds c2pool clean.

Author: frstrtr

.github/workflows/build.yml

@@ -639,11 +639,13 @@ 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')) }}
-    # Self-hosted fleet (VM217 c2pool-windows-217): internal-branch PRs and
-    # pushes run on the self-hosted Windows runner for speed; fork PRs fall
-    # back to GitHub-hosted windows-2022 so untrusted code never lands on the
-    # self-hosted box (defense-in-depth atop the repo fork-approval gate).
-    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' }}
+    # GitHub-hosted windows-2022 (ships cmake + MSVC/VS 2022 + the SDK).
+    # VM217 (c2pool-windows-217) is a bare runner with NO build toolchain yet;
+    # FOLLOW-UP: once vm-fleet installs VS 2022 Build Tools (C++ workload) +
+    # cmake + deps on VM217 and it builds c2pool clean, a follow-up PR flips
+    # this back to the fork-gated self-hosted expr. Linux+macOS already on the
+    # self-hosted fleet on this branch; Windows stays GitHub-hosted meanwhile.
+    runs-on: windows-2022
     steps:
       - uses: actions/checkout@v6