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.

pmb/helpers/repo_missing.py

@@ -108,7 +108,7 @@ def generate_output_format(arch, pkgnames):
     """
     ret = []
     for pkgname in pkgnames:
-        entry = pmb.helpers.package.get(pkgname, arch, True)
+        entry = pmb.helpers.package.get(pkgname, arch, True, try_other_arches=False)
         ret += [
             {
                 "pkgname": entry["pkgname"],