hack around setup-msys2 not liking CLANGARM64

.github/workflows/main.yml

@@ -43,7 +43,7 @@ jobs:
 
       - uses: msys2/setup-msys2@v2
         with:
-          msystem: ${{ matrix.msystem }}
+          msystem: ${{ contains(matrix.msystem, 'ARM64') && 'MSYS' || matrix.msystem }}
           install: VCS base-devel binutils pactoys ${{ matrix.toolchain }}
           update: true
           release: ${{ runner.arch != 'ARM64' }}
@@ -77,6 +77,12 @@ jobs:
         id: build
         run: |
           cd /C/_
+          if [[ "$MSYSTEM" != "${{ matrix.msystem }}" ]]; then
+            MSYSTEM=${{ matrix.msystem }}
+            set +e
+            . shell ${MSYSTEM,,}
+            set -e
+          fi
           MINGW_ARCH=${{ matrix.msystem }} ./.ci/ci-build.sh
 
       - name: "Upload binaries"