Use NuGet.exe for packing

The dotnet pack command can't handle nuspec files (or at least I can't
find the appropriate documentation for it). As such contining to use it
for creating NuGet packages for now.

build/scripts/build-utils.ps1

@@ -107,6 +107,32 @@ function Exec-Script([string]$script, [string]$scriptArgs = "") {
     Exec-Command "powershell" "-noprofile -executionPolicy RemoteSigned -file `"$script`" $scriptArgs"
 }
 
+# Ensure that NuGet is installed and return the path to the 
+# executable to use.
+function Ensure-NuGet() {
+    $nugetVersion = Get-ToolVersion "nugetExe"
+    $toolsDir = Join-Path $binariesDir "Tools"
+    Create-Directory $toolsDir
+
+    $destFile = Join-Path $toolsDir "NuGet.exe"
+    $versionFile = Join-Path $toolsDir "NuGet.exe.version"
+
+    # Check and see if we already have a NuGet.exe which exists and is the correct
+    # version.
+    if ((Test-Path $destFile) -and (Test-Path $versionFile)) {
+        $scratchVersion = Get-Content $versionFile
+        if ($scratchVersion -eq $nugetVersion) {
+            return $destFile
+        }
+    }
+
+    Write-Host "Downloading NuGet.exe"
+    $webClient = New-Object -TypeName "System.Net.WebClient"
+    $webClient.DownloadFile("https://dist.nuget.org/win-x86-commandline/v$nugetVersion/NuGet.exe", $destFile)
+    $nugetVersion | Out-File $versionFile
+    return $destFile
+}
+
 # Ensure the proper SDK in installed in our %PATH%. This is how MSBuild locates the 
 # SDK. Returns the location to the dotnet exe
 function Ensure-DotnetSdk() {

build/scripts/build.ps1

@@ -179,7 +179,9 @@ function Make-BootstrapBuild() {
 }
 
 function Build-Artifacts() { 
-    Run-MSBuild "Roslyn.sln" "/p:DeployExtension=false"
+    if ($build) { 
+        Run-MSBuild "Roslyn.sln" "/p:DeployExtension=false"
+    }
 
     if ($buildAll) {
         Build-ExtraSignArtifacts
@@ -281,6 +283,7 @@ function Build-NuGetPackages() {
         $buildArgs = '/p:SkipReleaseVersion=true /p:SkipPreReleaseVersion=true'
     }
 
+    Ensure-NuGet | Out-Null
     Run-MSBuild "src\NuGet\NuGet.proj" $buildArgs
 }
 
@@ -625,7 +628,7 @@ try {
         $bootstrapDir = Make-BootstrapBuild
     }
 
-    if ($build) {
+    if ($build -or $pack) {
         Build-Artifacts
     }