ci(windows): use shell: pwsh (PowerShell 7) — fixes GITHUB_ENV BOM corruption

Windows PowerShell 5.1's Out-File -Encoding utf8 prepends a UTF-8 BOM, corrupting the first
GITHUB_ENV line so CARGO_WORKSPACE_DIR silently never got set -> windows-reactor build.rs panic
-> CI build failed (runs 8765/8768). pwsh 7 writes UTF-8 without a BOM. Installed PowerShell 7.6.2
MSI on the runner and put C:\Program Files\PowerShell\7 on the daemon wrapper PATH so jobs find
pwsh; switched all windows.yml steps to shell: pwsh. (Reproduced locally with CARGO_WORKSPACE_DIR
set: the build is green in 2m37s — the BOM was the only issue.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/ci/setup-windows-runner.ps1

@@ -93,7 +93,7 @@ $env:RUSTUP_HOME = "C:\Users\Public\.rustup"
 $env:CMAKE_POLICY_VERSION_MINIMUM = "3.5"
 $env:LIBCLANG_PATH = "C:\Program Files\LLVM\bin"
 $env:FFMPEG_DIR = "C:\Users\Public\ffmpeg"
-$env:PATH = "C:\Users\Public\.cargo\bin;C:\nvm4w\nodejs;C:\Program Files\NASM;C:\Program Files\CMake\bin;C:\Program Files\LLVM\bin;C:\Users\Public\ffmpeg\bin;" + $env:PATH
+$env:PATH = "C:\Program Files\PowerShell\7;C:\Users\Public\.cargo\bin;C:\nvm4w\nodejs;C:\Program Files\NASM;C:\Program Files\CMake\bin;C:\Program Files\LLVM\bin;C:\Users\Public\ffmpeg\bin;" + $env:PATH
 Set-Location "C:\Users\Public\act-runner"
 # cmd-level redirect (>>, 2>&1) keeps the daemon's native stderr out of PowerShell's error stream.
 & cmd /c "act_runner.exe daemon --config config.yaml >> runner.log 2>&1"