MSYS2 CI: use pacboy and select ucrt64

Author: mgeier

.github/workflows/pd-externals.yml

@@ -54,11 +54,13 @@ jobs:
       - name: Install MSYS2
         uses: msys2/setup-msys2@v2
         with:
+          msystem: ucrt64
           release: false
           install: >-
             make
             unzip
-            mingw-w64-x86_64-gcc
+          pacboy: >-
+            gcc:p
       - name: Install Pd
         run: |
           wget -q -O Pd.zip http://msp.ucsd.edu/Software/pd-0.51-3.msw.zip