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.

msys2 hack:

https://stackoverflow.com/questions/76079590/error-plat-name-must-be-one-of-win32-win-amd64-win-arm32-win-arm6

scripts/msys2-build

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

scripts/msys2-install-deps

@@ -1,4 +1,4 @@
 #!/bin/bash
 
-pacman -S --needed --noconfirm git mingw-w64-ucrt-x86_64-{toolchain,pkgconf,zstd,lz4,xxhash,openssl,python,cython,python-setuptools,python-wheel,python-pkgconfig,python-packaging,python-msgpack,python-pip}
+pacman -S --needed --noconfirm git mingw-w64-ucrt-x86_64-{toolchain,pkgconf,zstd,lz4,xxhash,openssl,python,cython,python-setuptools,python-wheel,python-build,python-pkgconfig,python-packaging,python-msgpack,python-pip}
 pip install pyinstaller==5.13.2