What is the easiest way to install the latest postgis_tiger_geocoder?
Well, they don't make this easy because it's part of core. It's also not fun that this is hard coded into the extension. You can either


Follow the instructions in the official FAQ by extracting from the .zip
Rebuild the entirety of PostGIS which is a monstrously fun task -- though it is totally doable. There is no easy way to build just the extension. The extensions build after the documentation and the rest of PostGIS. Good luck and god speed.
Download PostGIS-Scripts from the PDPG apt archive. Note, the file you want is labeled postgresql-$majorversionpg-postgis-$majorversionpostgis-scripts_$fullversionpostgis with the version of PostgreSQL and PostGIS. Do not download the transition packages that lack the $majorversion portion of the name.


After you download extract the files with dpkg -X justDownloadedArchive.deb ./out
Copy out/*geocoder* to your extension folder (/usr/share/postgresql/*/extension/) where * is some version number.