Fix 7zip ARM64: suppress post_install error with SilentlyContinue

The Remove-Item error on 7zr.exe happens on ARM64 regardless of user
context (packer user, not just SYSTEM). Temporarily set ErrorActionPreference
to SilentlyContinue and convert all output streams to strings so the
error doesn't propagate to PowerShell's exit code.

[build images]

scripts/bootstrap.ps1

@@ -242,9 +242,23 @@ function Install-Ruby {
 }
 
 function Install-7zip {
-  # With Packer (WinRM), scoop runs as the packer user, not SYSTEM.
-  # The ARM64 Remove-Item error only happens under SYSTEM context (Azure Run Command).
-  Install-Scoop-Package 7zip -Command 7z
+  if (Which 7z) {
+    return
+  }
+
+  Write-Output "Installing 7zip (via Scoop)..."
+  # On ARM64, scoop's 7zip post_install tries to Remove-Item 7zr.exe which is
+  # locked during extraction. This error is non-fatal (7zip installs fine).
+  # Temporarily allow errors so it doesn't kill the bootstrap.
+  $prevErrorPref = $ErrorActionPreference
+  $ErrorActionPreference = "SilentlyContinue"
+  scoop install 7zip *>&1 | ForEach-Object { "$_" } | Write-Host
+  $ErrorActionPreference = $prevErrorPref
+  Refresh-Path
+
+  if (-not (Which 7z)) {
+    throw "7zip installation failed"
+  }
 }
 
 function Install-Make {