Use ccache in reprotest (or debrebuild)

Related to #27 (closed), I wonder if the reprotest could also use ccache? Big packages like MariaDB sometime fail to build in less than 2h on the relatively slow Salsa-CI runners, and building it twice in 2h will just never happen, thus the reprotest for MariaDB always fails (see e.g. https://salsa.debian.org/mariadb-team/mariadb-10.3/-/jobs/134690).

With ccache, it might be able to finish in well under an hour.