Fix shell specification for Mingw

Author: tobiasdiez

.github/workflows/ci.yml

@@ -166,6 +166,7 @@ jobs:
           path-type: inherit
 
       - name: Build
+        shell: msys2 {0}
         run: |
           # Install build dependencies manually as workaround for https://github.com/astral-sh/uv/issues/1516
           uv pip install \
@@ -180,6 +181,7 @@ jobs:
           uv sync --frozen --inexact --no-build-isolation -v --no-editable --config-settings=builddir=builddir
 
       - name: Test
+        shell: msys2 {0}
         run: |
           uv run --frozen ./sage -t --all -p4 || true