github: Switch test-ffmpeg over to the msys2 environment ucrt64

The mingw64 environment is deprecated, which the setup-msys2 action
warns about.

We don't really use the host environment's tools anyway so the
choice doesn't matter. Ideally we maybe would use the msys
environment instead, to avoid accidentally relying on build tools
from there. But if building in the msys environment, ffmpeg things
we're building for msys as opposed to native mingw. (This can
probably be avoided by specifying --target-os though.)

.github/workflows/build.yml

@@ -753,7 +753,7 @@ jobs:
           git config --global core.autocrlf false
       - uses: msys2/setup-msys2@v2
         with:
-          msystem: mingw64
+          msystem: ucrt64
           install: >-
             unzip
             make