repo_missing: set try_other_arches to False

Don't attempt to find packages of other arches in repo_missing code.
This leads to unexpected results in general (we want to check if a
binary package exists for only one specific architecture!) - and in case
of master_staging_systemd, it current causes errors because pmbootstrap
tries to fetch armhf, riscv etc. APKINDEXes for which we don't build the
staging repositories.