Add a dedicated Pre-Release On-Demand workflow that builds from the dev
branch (branch input, default dev) and publishes a GitHub release marked
as --prerelease, with an optional custom version suffix input.

Extend tools/gen_version.py with a 'pre-release' BUILD_TYPE
(0.0.1-pre-<hash>) and a CUSTOM_VERSION env var that appends a custom
string to the base version (0.0.1-umtx2-test), which also shows in the
PS5 homescreen app title (param.json). build_release.sh now passes
CUSTOM_VERSION into the Docker build.