You'll probably have to install from source. The documentation describes that:
git clone https://github.com/citusdata/pg_cron.git
cd pg_cron
# Ensure pg_config is in your path, e.g.
export PATH=/usr/pgsql-12/bin:$PATH
make && sudo PATH=$PATH make install
You will need GNU make, a C compiler and the PostgreSQL headers installed; other dependencies have to be found out the hard way, as the documentation is silent about them.