ci: rollback to non-broken msys2-runtime

It appears that msys2-runtime 3.4.10-5 is the cause of `make prove`
breaking in the Windows build. This change forces an earlier
msys2-runtime package to be installed.

Author: jpgrayson

.github/workflows/ci.yml

@@ -228,6 +228,10 @@ jobs:
           update: true
           install: diffutils make
           path-type: inherit
+      - name: Downgrade msys2 runtime
+        shell: msys2 {0}
+        run: |
+          pacman --noconfirm -U https://repo.msys2.org/msys/x86_64/msys2-runtime-3.4.10-4-x86_64.pkg.tar.zst
       - name: Test
         shell: msys2 {0}
         env: