ci(windows): bootstrap rust toolchain for runner user; install rust machine-wide in future provisions

ci/runners/windows/provision.ps1

@@ -54,6 +54,7 @@ choco install -y --no-progress `
     git python311 nuget.commandline 7zip cmake ninja `
     nodejs-lts powershell-core dotnet-sdk
 Add-MachinePath 'C:\Program Files\Git\cmd'
+Add-MachinePath 'C:\Program Files\Git\bin'   # bash.exe + posix tools (sed, find, etc.)
 Add-MachinePath 'C:\Python311'
 Add-MachinePath 'C:\Python311\Scripts'
 Add-MachinePath 'C:\Program Files\nodejs'
@@ -81,11 +82,21 @@ if (-not $vsPresent) {
 }
 
 # --- 3. Rust (stable 1.75 + nightly-2023-10-13 with i686 target) ---
-if (-not (Get-Command rustup -ErrorAction SilentlyContinue)) {
-    Write-Host '==> Installing rustup'
+# Install machine-wide so the act_runner service (LocalSystem) and any admin user
+# share the same toolchain registry. Without this, rustup state lives in the
+# installing user's profile and LocalSystem ends up with no default toolchain.
+$rustupHome = 'C:\rustup'
+$cargoHome  = 'C:\cargo'
+[Environment]::SetEnvironmentVariable('RUSTUP_HOME', $rustupHome, 'Machine')
+[Environment]::SetEnvironmentVariable('CARGO_HOME',  $cargoHome,  'Machine')
+$env:RUSTUP_HOME = $rustupHome
+$env:CARGO_HOME  = $cargoHome
+Add-MachinePath "$cargoHome\bin"
+
+if (-not (Test-Path "$cargoHome\bin\rustup.exe")) {
+    Write-Host '==> Installing rustup (machine-wide at C:\rustup, C:\cargo)'
     Invoke-WebRequest -Uri 'https://win.rustup.rs/x86_64' -OutFile "$env:TEMP\rustup-init.exe"
     & "$env:TEMP\rustup-init.exe" -y --default-toolchain none --profile minimal
-    Add-MachinePath "$env:USERPROFILE\.cargo\bin"
 }
 rustup toolchain install $RUST_VERSION --profile minimal --component rustfmt
 rustup target add --toolchain $RUST_VERSION x86_64-pc-windows-msvc