github/workflows: Improve MSYS2-based CI builds.

Install the mingw variant of Python since it behaves more like a 'real'
Windows CPython than the msys2 variant: os.name == 'nt', not 'posix'.  Note
that os.sep is still '/' though so we don't actually need to skip the
import_file test.  This way one single Python version can be used both for
running run-tests.py and getting the expected test output.

Signed-off-by: stijn <stijn@ignitron.net>

.github/workflows/ports_windows.yml

@@ -108,16 +108,6 @@ jobs:
       run:
         shell: msys2 {0}
     steps:
-    - name: Get Python path
-      id: python_path
-      shell: python
-      run: |
-        import os
-        import sys
-        output = f"python={os.fspath(sys.executable)}"
-        print(output)
-        with open(os.environ["GITHUB_OUTPUT"], "w") as f:
-            f.write(output)
     - uses: msys2/setup-msys2@v2
       with:
         msystem: ${{ matrix.sys }}
@@ -126,7 +116,7 @@ jobs:
           make
           mingw-w64-${{ matrix.env }}-gcc
           pkg-config
-          python3
+          mingw-w64-${{ matrix.env }}-python3
           git
           diffutils
     - uses: actions/checkout@v4
@@ -138,8 +128,7 @@ jobs:
       run: make -C ports/windows -j2 VARIANT=${{ matrix.variant }}
     - name: Run tests
       id: test
-      # msys python breaks tests so we need to use "real" windows python
-      run: MICROPY_CPYTHON3=$(cygpath "${{ steps.python_path.outputs.python }}") make -C ports/windows test_full VARIANT=${{ matrix.variant }}
+      run: make -C ports/windows test_full VARIANT=${{ matrix.variant }}
     - name: Print failures
       if: failure() && steps.test.conclusion == 'failure'
       working-directory: tests