build-python: Fix rebuilding with an already checked out directory

After applying patches (with plain 'patch' instead of 'git am'), we
need to reset the tree to be able to check out the targeted version
again.

build-python.sh

@@ -120,6 +120,7 @@ fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_PYTHON" ]; then
     cd cpython-mingw
     [ -z "$SYNC" ] || git fetch
+    git reset --hard
     git checkout $PYTHON_VERSION_MINGW
     cat ../patches/python/*.patch | patch -Nup1
     autoreconf -vfi