use correct path

Author: bvogelzang

appveyor.yml

@@ -21,7 +21,7 @@ install:
   - gem --version
 
   # update all packages on the system
-  - pacman -Syu
+  - c:/msys64/usr/bin/pacman -Syu
 
   # prevent freetds to link to wrong ws2_32 lib on i686-w64-mingw32
   - c:/msys64/usr/bin/rm /usr/lib/w32api/libws2_32.a