Stop continuously building the proto (#10771)

eng/Build.ps1

@@ -163,7 +163,7 @@ function Update-Arguments() {
             $script:bootstrap = $True
         }
     } else {
-        if (-Not (Test-Path "$ArtifactsDir\Bootstrap\fsc\fsc.dll") -or (Test-Path "$ArtifactsDir\Bootstrap\fsc\fsc.runtimeconfig.json")) {
+        if (-Not (Test-Path "$ArtifactsDir\Bootstrap\fsc\fsc.exe") -or (Test-Path "$ArtifactsDir\Bootstrap\fsc\fsc.runtimeconfig.json")) {
             $script:bootstrap = $True
         }
     }