Revert build script fixes

Revert some changes to the build scripts but try and retain workaround for Azure Pipelines.

Author: martincostello

Build.ps1

@@ -5,6 +5,11 @@ param(
     [Parameter(Mandatory = $false)][switch] $SkipTests
 )
 
+# These make CI builds faster
+$env:DOTNET_MULTILEVEL_LOOKUP = "0"
+$env:DOTNET_SKIP_FIRST_TIME_EXPERIENCE = "true"
+$env:NUGET_XMLDOC_MODE = "skip"
+
 $ErrorActionPreference = "Stop"
 $ProgressPreference = "SilentlyContinue"
 
@@ -52,13 +57,33 @@ else {
 
 if ($installDotNetSdk -eq $true) {
 
-    $installScript = Join-Path (Join-Path "$(Convert-Path "$PSScriptRoot")" ".dotnetcli") "install.ps1"
-    [Net.ServicePointManager]::SecurityProtocol = [Net.ServicePointManager]::SecurityProtocol -bor "Tls12"
-    Invoke-WebRequest "https://dot.net/v1/dotnet-install.ps1" -OutFile $installScript -UseBasicParsing
-    & $installScript -Version "$dotnetVersion"
+    if (($null -ne $env:TF_BUILD)) {
+        $env:DOTNET_INSTALL_DIR = Join-Path $env:ProgramFiles "dotnet"
+    } else {
+        $env:DOTNET_INSTALL_DIR = Join-Path "$(Convert-Path "$PSScriptRoot")" ".dotnetcli"
+    }
+
+    $sdkPath = Join-Path $env:DOTNET_INSTALL_DIR "sdk\$dotnetVersion"
+
+    if (($null -ne $env:TF_BUILD) -or (!(Test-Path $sdkPath))) {
+        if (!(Test-Path $env:DOTNET_INSTALL_DIR)) {
+            mkdir $env:DOTNET_INSTALL_DIR | Out-Null
+        }
+        $installScript = Join-Path $env:DOTNET_INSTALL_DIR "install.ps1"
+        [Net.ServicePointManager]::SecurityProtocol = [Net.ServicePointManager]::SecurityProtocol -bor "Tls12"
+        Invoke-WebRequest "https://dot.net/v1/dotnet-install.ps1" -OutFile $installScript -UseBasicParsing
+        & $installScript -Version "$dotnetVersion" -InstallDir "$env:DOTNET_INSTALL_DIR" -NoPath
+    }
+}
+else {
+    $env:DOTNET_INSTALL_DIR = Split-Path -Path (Get-Command dotnet.exe).Path
 }
 
-$dotnet = "dotnet"
+$dotnet = Join-Path "$env:DOTNET_INSTALL_DIR" "dotnet.exe"
+
+if (($installDotNetSdk -eq $true) -And ($null -eq $env:TF_BUILD)) {
+    $env:PATH = "$env:DOTNET_INSTALL_DIR;$env:PATH"
+}
 
 function DotNetPack {
     param([string]$Project)