fix(xcat-dep): publish the riscv64 index the build stages

In publish mode with no --expect-arch, the expected set is discovered by
scanning the staged tree, and that scan admitted a hardcoded amd64 or
ppc64el. A staged riscv64 tree was therefore dropped, and only expected
architectures get a binary-<arch> index written and named in the Release
file, so the packages built for riscv64 never reached the repository. The
scan and the verification probe now use the architecture set the rest of
the script already reads from supported_arches().

sbuild-all.pl

@@ -869,7 +869,7 @@ sub resolve_expect_arches {
             for my $d (glob("$staging/$cn/*")) {
                 next unless -d $d;
                 my $a = basename($d);
-                $u{$a} = 1 if $a =~ /^(amd64|ppc64el)$/;
+                $u{$a} = 1 if is_supported_arch($a);
             }
         }
         print "  expected arches (from the staged set): " . join(' ', sort keys %u) . "\n";
@@ -926,7 +926,7 @@ sub verify_assembled_repo {
         # pure verify_repo_arches can report both directions: expected-but-absent and present-but-
         # unexpected (a stale arch left behind in the tree).
         my %native;
-        for my $a (do { my %s = map { $_ => 1 } (@expected, qw(amd64 ppc64el)); sort keys %s }) {
+        for my $a (do { my %s = map { $_ => 1 } (@expected, supported_arches()); sort keys %s }) {
             my $idx = "$adir/dists/$cn/main/binary-$a/Packages";
             $native{$a} = 0;
             next unless -f $idx;
@@ -1438,7 +1438,7 @@ C<--skip-tarball>.
 
 =over 4
 
-=item B<--arch> C<amd64|ppc64el>
+=item B<--arch> C<amd64|ppc64el|riscv64>
 
 Host architecture. Default: C<dpkg --print-architecture>.
 
@@ -1552,7 +1552,7 @@ swap it onto C<--apt-dir> atomically. B<A build run does not publish unless this
 arches can build concurrently without racing each other on the shared repo. Defaults to on for a run
 that builds nothing (C<--skip-build>), which is the finalization step. C<--no-publish> forces it off.
 
-=item B<--expect-arch> C<< amd64|ppc64el >>
+=item B<--expect-arch> C<< amd64|ppc64el|riscv64 >>
 
 The architecture set the published repo must serve, stated explicitly. Repeatable, and each value may
 be a space/comma list (C<--expect-arch "amd64 ppc64el">). Used by the gate: an expected arch with no