Revert msys2/setup-msys2 to v2.31.0

v2.31.1 fails to verify MSYS2 package database PGP signatures on
GitHub-hosted Windows runners ("signature from Christoph Reiter
... is unknown trust" for clangarm64/mingw32/mingw64/ucrt64/clang64/msys),
which breaks the Windows ARM64 (and x64) jobs at the pacman -Syuu step.
Pinning back to v2.31.0 until upstream ships a fix.

.github/workflows/ci.yml

@@ -132,7 +132,7 @@ jobs:
           toolchain: ${{ env.RUST_TOOLCHAIN }}
 
       - name: Set up MSYS2
-        uses: msys2/setup-msys2@e9898307ac31d1a803454791be09ab9973336e1c # v2.31.1
+        uses: msys2/setup-msys2@cafece8e6baf9247cf9b1bf95097b0b983cc558d # v2.31.0
         with:
           msystem: ${{ matrix.msystem }}
           update: true

.github/workflows/release.yml

@@ -274,7 +274,7 @@ jobs:
           toolchain: ${{ env.RUST_TOOLCHAIN }}
 
       - name: Set up MSYS2
-        uses: msys2/setup-msys2@e9898307ac31d1a803454791be09ab9973336e1c # v2.31.1
+        uses: msys2/setup-msys2@cafece8e6baf9247cf9b1bf95097b0b983cc558d # v2.31.0
         with:
           msystem: ${{ matrix.msystem }}
           update: true