PostGIS raster is part of the standard postgresql-N-postgis-3 package on Debian/Ubuntu; no extra package is required. Pass -DRASTER=ON to CMake:

cmake -DRASTER=ON ..
make -j$(nproc)
sudo make install

To include raster in the CI coverage build alongside other optional families, add -DRASTER=ON to the cmake invocation in .github/workflows/pgversion.yml.