build: osxpkg: build lxml (#5687)

lxml wheels are built with an old SDK, which breaks osxpkg notarization:

```
	* error for path "dvc-2.0.7.pkg/dvc-2.0.7.pkg Contents/Payload/usr/local/lib/dvc/lxml/etree.cpython-37m-darwin.so": The binary uses an SDK older than the 10.9 SDK.
```

.github/workflows/packages.yaml

@@ -88,7 +88,10 @@ jobs:
         ruby-version: 2.4
     - name: Install deps
       run: |
-        pip install .[all]
+        # available lxml (webdav dependency) wheels are built with an old
+        # SDK, which breaks osxpkg notarization. Building from scratch
+        # instead.
+        pip install .[all] --no-binary lxml
         pip install -r scripts/build-requirements.txt
         gem install --no-document fpm
     - name: Build