github: Remove a redundant "update: false" for setup-msys2

"update: false" is the default, and we don't specify it elsewhere.

.github/workflows/build.yml

@@ -547,7 +547,6 @@ jobs:
       - uses: msys2/setup-msys2@v2
         with:
           msystem: mingw64
-          update: false
           install: >-
             unzip
             make