Merged PR 13221253: don't run msrustup at all within ADO

don't run msrustup at all within ADO

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#### AI description  (iteration 1)
#### PR Classification
This PR updates the build script to disable the execution of msrustup in ADO environments.

#### PR Summary
The pull request modifies `/build.ps1` to ensure that msrustup is not run when the build is executed within ADO. It introduces an early exit when the ADO flag is set and simplifies the rust toolchain selection and update logic.
- `build.ps1`: Adds a conditional branch that breaks out when `$usingADO` is true, preventing msrustup execution.
- `build.ps1`: Removes the fallback assignment of `$rustup` to `echo` and adjusts the update condition for clarity and proper toolchain selection.
<!-- GitOpsUserAgent=GitOps.Apps.Server.pullrequestcopilot -->

Author: SteveL-PowerShell

build.ps1

@@ -155,16 +155,17 @@ function Find-LinkExe {
 }
 
 $channel = 'stable'
-if ($null -ne (Get-Command msrustup -CommandType Application -ErrorAction Ignore)) {
+$rustup = 'echo'
+if ($usingADO) {
+    return
+} elseif ($null -ne (Get-Command msrustup -CommandType Application -ErrorAction Ignore)) {
     $rustup = 'msrustup'
     $channel = 'ms-stable'
     if ($architecture -eq 'current') {
         $env:MSRUSTUP_TOOLCHAIN = "$architecture"
     }
 } elseif ($null -ne (Get-Command rustup -CommandType Application -ErrorAction Ignore)) {
         $rustup = 'rustup'
-} else {
-    $rustup = 'echo'
 }
 
 if ($null -ne $packageType) {
@@ -185,7 +186,7 @@ if ($null -ne $packageType) {
             Remove-Item temp:/rustup-init.exe -ErrorAction Ignore
         }
     }
-    elseif (!$usingADO) {
+    else {
         Write-Verbose -Verbose "Rust found, updating..."
         & $rustup update
     }