appveyor don't install nsis in debug

.appveyor.yml

@@ -11,7 +11,7 @@ install:
   - call "%ProgramFiles(x86)%\Microsoft Visual Studio 12.0\VC\vcvarsall.bat" x86
   - set PATH=C:\Qt\Tools\QtCreator\bin;C:\Qt\5.4\msvc2013_opengl\bin;%PATH%
   - mkdir %LOCALAPPDATA%\QtProject && copy test\qtlogging.ini %LOCALAPPDATA%\QtProject\
-  - cinst nsis -y -installArgs /D="%programfiles(x86)%\NSIS"
+  - if "%CONFIG%" EQU "installer" ( cinst nsis -y -installArgs /D="%programfiles(x86)%\NSIS" )
 
 build_script:
   - C:\Qt\5.4\msvc2013_opengl\bin\qmake -r CONFIG-=debug_and_release CONFIG+=%CONFIG% CONFIG+=WarningsAsErrorsOn CONFIG+=jombuild qgroundcontrol.pro