github: Stop building in the mingw64 environment in msys2

These builds fail regularly, under mysterious circumstances.
("The hosted runner lost communication with the server. Anything
in your workflow that terminates the runner process, starves it
for CPU/Memory, or blocks its network access can cause this error.")

This usually seems to happen around the build-libcxx.sh step.

.github/workflows/build.yml

@@ -329,9 +329,8 @@ jobs:
             llvm-mingw-*.tar.xz
           retention-days: 7
 
-  # Test building the toolchain on msys2 (in the mingw64 and clang64
-  # environments). The binaries built here rely on the runtime libraries from
-  # the host environment (libstdc++ or libc++).
+  # Test building the toolchain on msys2. The binaries built here rely on the
+  # runtime libraries from the host environment (libstdc++ or libc++).
   msys2:
     needs: [prepare]
     runs-on: windows-latest
@@ -342,7 +341,6 @@ jobs:
       fail-fast: false
       matrix:
         sys:
-          - mingw64
           - clang64
     steps:
       - uses: msys2/setup-msys2@v2