Correct test for MSYS2/Cygwin in AppVeyor script

There is no "compiler" environment variable, use TOOLSET instead.

appveyor.yml

@@ -62,7 +62,7 @@ before_build:
     $env:PATH = $env:PATH -replace "C:\\Program Files\\Git\\usr\\bin",""
     if (-not (Test-Path env:wxUSE_STL)) { $env:wxUSE_STL = '0' }
     if (-not (Test-Path env:wxUSE_WEBVIEW_EDGE)) { $env:wxUSE_WEBVIEW_EDGE = '0' }
-    if (($env:compiler -ne "msys2") -and ($env:compiler -ne "cygwin")) {
+    if (($env:TOOLSET -ne "msys2") -and ($env:TOOLSET -ne "cygwin")) {
       $txt = gc include\wx\msw\setup.h
       Write-Output $txt |
       %{$_ -replace "define wxUSE_STL 0", "define wxUSE_STL $env:wxUSE_STL"} |