Nightly MacOS test runs were failing due to the updated integration test
structure recently introduced to the project. Part of these changes
addresses that situation.

The other part of the changes introduces an mdbook nightly build. This
does not publish the documentation, it only checks to make sure that
mdbook+plugins can install and build the documentation properly. I
believe it was necessary because the only time documentation is built is
when a release happens, and if documentation generation fails for
whatever reason, we find out only after a release has been published.
The hope is that this should catch any errors before we release.

NOTE: This PR wants to be merged into tcdi/plrust:main directly, as it
is only CI changes and the nightly updates can be applied immediately
instead of waiting for another plrust release.

Addresses: #377

When plrust goes through a release cycle, occasionally (read: pretty
much "almost" every time) we forget to update the hard-coded pgrx
version in the `plrust/build` script due to the fact that our automated
release tooling does not consider nor modify the script file. With the
changes in this PR, we programmatically get the pgrx version using
`cargo tree` and some unix utils magic so that nobody has to think about
updating that value ever again upon release.