[appveyor] kill bash before second try

Ok, just waiting for bash termination doesn't works, so lets kill it.

appveyor.yml

@@ -30,8 +30,8 @@ install:
 - c:\msys64\usr\bin\bash -lc "pacman --noconfirm -Rns gcc-fortran gcc mingw-w64-{i686,x86_64}-gcc-ada mingw-w64-{i686,x86_64}-gcc-objc"
 # update runtime
 - c:\msys64\usr\bin\bash -lc "pacman --noconfirm -Syuu"
-# Sleep for 5 seconds before next try?
-- ping -n 5 127.0.0.1 > nul
+# Kill bash before next try
+- taskkill /T /F /IM bash.exe
 # update packages and install required
 - c:\msys64\usr\bin\bash -lc "pacman --noconfirm -Syuu ${MSYS_PACKAGES}"