fix msys build

build the sdist and wheel using "build", avoid using setup.py.

use "pip install -e ." before invoking pyinstaller, so we have
the _version.py built by setuptools_scm and also it compiled the
extension modules, so pyinstaller can pick them up.

scripts/msys2-build

@@ -1,5 +1,8 @@
 #!/bin/bash
 
-python setup.py build_ext --inplace
-python setup.py bdist_wheel
+# build sdist and wheel in dist/...
+python -m build
+
+# build borg.exe
+pip install -e .
 pyinstaller -y scripts/borg.exe.spec