Use cached 'dotnet' in AppVeyor (#4278)

build.psm1

@@ -1192,8 +1192,30 @@ function Start-PSBootstrap {
             }
         }
 
-        $DotnetArguments = @{ Channel=$Channel; Version=$Version; NoSudo=$NoSudo }
-        Install-Dotnet @DotnetArguments
+        $dotNetExists = precheck 'dotnet' $null
+        $dotNetVersion = [string]::Empty
+        if($dotNetExists) {
+            $dotNetVersion = (dotnet --version)
+        }
+
+        if(!$dotNetExists -or $dotNetVersion -ne $dotnetCLIRequiredVersion -or $Force.IsPresent) {
+            if($Force.IsPresent) {
+                log "Installing dotnet due to -Force."
+            }
+            elseif(!$dotNetExistis) {
+                log "dotnet not present.  Installing dotnet."
+            }
+            else {
+                log "dotnet out of date ($dotNetVersion).  Updating dotnet."
+            }
+
+            $DotnetArguments = @{ Channel=$Channel; Version=$Version; NoSudo=$NoSudo }
+            Install-Dotnet @DotnetArguments
+        }
+        else
+        {
+            log "dotnet is already installed.  Skipping installation."
+        }
 
         # Install for Windows
         if ($IsWindows) {

tools/appveyor.psm1

@@ -3,6 +3,12 @@ $repoRoot = Join-Path $PSScriptRoot '..'
 $script:administratorsGroupSID = "S-1-5-32-544"
 $script:usersGroupSID = "S-1-5-32-545"
 
+$dotNetPath = "$env:USERPROFILE\Appdata\Local\Microsoft\dotnet"
+if(Test-Path $dotNetPath)
+{
+    $env:PATH = $dotNetPath + ';' + $env:PATH
+}
+
 Import-Module (Join-Path $repoRoot 'build.psm1')
 
 function New-LocalUser
@@ -225,7 +231,7 @@ function Invoke-AppVeyorInstall
     }
 
     Set-BuildVariable -Name TestPassed -Value False
-    Start-PSBootstrap -Force
+    Start-PSBootstrap -Confirm:$false
 }
 
 # A wrapper to ensure that we upload test results