appveyor.yml: try only updating msys2 core, not the rest of the packages

maybe it'll get a bit faster
diff --git a/appveyor.yml b/appveyor.yml
index 9b00d84..6250a91 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -30,7 +30,6 @@
 
 install:
   - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "pacman -Syu --noconfirm"'
-  - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "pacman -Su --noconfirm"'
   - C:\msys64\usr\bin\bash -lc "pacman --noconfirm -S mingw-w64-x86_64-ragel"
 
 build_script: