do not install NSIS for PRs

scripts/ps/appveyor_install.ps1

@@ -0,0 +1,21 @@
+#########################################################
+# Script to setup OpenRCT2 for building on AppVeyor
+#########################################################
+
+if ($env:ENCKEY)
+{
+    Write-Host "Downloading secure-file from NuGet" -ForegroundColor Cyan
+    nuget install secure-file -ExcludeVersion
+    secure-file\tools\secure-file -decrypt distribution\windows\code-sign-key-openrct2.org.pfx.enc -secret $env:ENCKEY
+}
+
+if (${env:OPENRCT2.ORG_TOKEN})
+{
+    Write-Host "Downloading NSIS from chocolatey"   -ForegroundColor Cyan
+    cinst nsis.portable --version=3.01-beta1 > $null
+
+    Write-Host "Downloading KillProcDLL for NSIS"   -ForegroundColor Cyan
+    curl "http://nsis.sourceforge.net/mediawiki/images/5/53/KillProcDll%26FindProcDll.zip" -OutFile nsisxtra.zip
+    7z x nsisxtra.zip > $null
+    cp FindProcDLL.dll "C:\ProgramData\chocolatey\lib\nsis.portable\tools\nsis-3.0b1\Plugins\x86-ansi"
+}