GH Actions: add caching for xmllint install on Windows

PR #180 added test runs against Windows, but these are terribly slow due to `choco install` for installing `xmllint` taking a while.

I imagine we could use `actions/cache` to cache the `xmllint` installation and retrieve from cache in most cases. To determine whether a new install is needed a version check is probably needed.

Note: this is for the `develop` (2.x) branch!