Standardize use of prepareMachine for ci cleanup (#71265)

Arcade has a concept of prepareMachine which is commonly (though not always) enabled alongside the -ci switch. This causes process cleanup when a build function exits. Roslyn was using this switch in some cases, but not in others, and had implemented a process cleanup with -ci was passed. In the VMR windows build, this causes roslyn to kill the host build process. Change this so that -ci no longer performs this behavior by default, and instead use -prepareMachine to have Arcade do this for us.

azure-pipelines-compliance.yml

@@ -38,6 +38,7 @@ steps:
   inputs:
     filePath: eng/build.ps1
     arguments: -ci
+               -prepareMachine
                -restore
                -build
                -configuration $(BuildConfiguration)

azure-pipelines-official.yml

@@ -180,6 +180,7 @@ stages:
       inputs:
         filePath: eng/build.ps1
         arguments: -ci
+                   -prepareMachine
                    -restore
                    -build
                    -pack

azure-pipelines-pr-validation.yml

@@ -122,6 +122,7 @@ stages:
       inputs:
         filePath: eng/build.ps1
         arguments: -ci
+                   -prepareMachine
                    -restore
                    -build
                    -pack

eng/build.ps1

@@ -799,10 +799,6 @@ catch {
   ExitWithExitCode 1
 }
 finally {
-  if ($ci) {
-    Stop-Processes
-  }
-
   if (Test-Path Function:\Unsubst-TempDir) {
     Unsubst-TempDir
   }

eng/cibuild.sh

@@ -16,4 +16,4 @@ scriptroot="$( cd -P "$( dirname "$source" )" && pwd)"
 echo "Building this commit:"
 git show --no-patch --pretty=raw HEAD
 
-. "$scriptroot/build.sh" --ci --restore --build --bootstrap --pack --publish --binaryLog "$@"
+. "$scriptroot/build.sh" --ci --prepareMachine --restore --build --bootstrap --pack --publish --binaryLog "$@"

eng/test-rebuild.ps1

@@ -6,6 +6,7 @@
 param(
   [string]$configuration = "Debug",
   [switch]$ci = $false,
+  [switch]$prepareMachine = $false,
   [switch]$useGlobalNuGetCache = $true,
   [switch]$noBuild = $false,
   [switch]$help)
@@ -33,7 +34,7 @@ try {
 
   if (-not $noBuild) {
     Write-Host "Building Roslyn"
-    Exec-Block { & (Join-Path $PSScriptRoot "build.ps1") -restore -build -bootstrap -ci:$ci -useGlobalNuGetCache:$useGlobalNuGetCache -configuration:$configuration -pack -binaryLog }
+    Exec-Block { & (Join-Path $PSScriptRoot "build.ps1") -restore -build -bootstrap -prepareMachine:$prepareMachine -ci:$ci -useGlobalNuGetCache:$useGlobalNuGetCache -configuration:$configuration -pack -binaryLog }
   }
 
   Subst-TempDir