Revert "github: Use msys instead of mingw64 for running test-toolchain"

This reverts commit 38847df.

This makes the "cmake" binary installed here behave as a native
windows cmake, not a unix/cygwin/msys one.

Author: mstorsjo

.github/workflows/build.yml

@@ -440,7 +440,7 @@ jobs:
     steps:
       - uses: msys2/setup-msys2@v2
         with:
-          msystem: msys
+          msystem: mingw64
           install: >-
             unzip
             make