appveyor: retry src update

git sometimes fails

appveyor.sh

@@ -6,4 +6,9 @@ git clone https://github.com/msys2/MINGW-packages
 git clone https://github.com/msys2/MSYS2-packages
 curl -o srcinfo.json -L 'https://github.com/msys2/msys2-web/releases/download/cache/srcinfo.json'
 
-python3 -u update-srcinfo.py srcinfo.json MINGW-packages MSYS2-packages
+n=0
+until [ $n -ge 5 ]
+do
+   python3 -u update-srcinfo.py srcinfo.json MINGW-packages MSYS2-packages && break || sleep 10
+   n=$[$n+1]
+done