Improve "Signature verification" scripts to make them easier to run

The Signature verification section of the release prep template includes running some scripts to verify the release was correctly signed. However those scripts currently require manually installing/building some dependencies, setting some environment variables and entering the build directory before running those scripts.

We could improve that so that a single makefile target (or command in the tools/signing directory) can be used to start all the verification scripts without requiring manual setup.

The script tools/signing/check_file_counts could also be added to the verification process (and fixed if it's currently broken).