[Ignore] Install pwsh when needed in release builds (#1202)

Author: rjmholt

.vsts-ci/templates/release-general.yml

@@ -1,4 +1,19 @@
 steps:
+- powershell: |
+    Write-Host "Installing pwsh..."
+    if (Get-Command pwsh -ErrorAction Ignore)
+    {
+        Write-Host "pwsh already installed, skipping"
+        return
+    }
+    $powerShellPath = Join-Path -Path $env:AGENT_TEMPDIRECTORY -ChildPath 'powershell'
+    Invoke-WebRequest -Uri https://raw.githubusercontent.com/PowerShell/PowerShell/master/tools/install-powershell.ps1 -outfile ./install-powershell.ps1
+    ./install-powershell.ps1 -Destination $powerShellPath
+    $vstsCommandString = "vso[task.setvariable variable=PATH]$powerShellPath;$env:PATH"
+    Write-Host "sending " + $vstsCommandString
+    Write-Host "##$vstsCommandString"
+  displayName: Install PowerShell Core
+
 - pwsh: |
     Get-ChildItem -Path env:
   displayName: Capture environment