Keep the AS list when the asupdate generator fails (closes #173)

make asupdate piped the generator into gzip, so the target saw gzip's
exit status. A failed download replaced pkg/asinfo/asdata.json.gz with
an empty file and the target still exited 0.

The target now runs the new script/asupdate. It writes the generator's
output to a temporary file and checks that the generator succeeded and
wrote at least one AS. Only then does it compress the list and move it
into place. Otherwise it leaves the file alone and exits non-zero.

The generator reads as.csv from ipverse/as-metadata, the source's new
name. The list it builds there is unchanged, so no new list is
committed.

Model: opus-5-5

cmd/asinfo-gen/main.go

@@ -15,7 +15,7 @@ import (
 	"strings"
 )
 
-const asCsvURL = "https://github.com/ipverse/asn-info/raw/refs/heads/master/as.csv"
+const asCsvURL = "https://github.com/ipverse/as-metadata/raw/refs/heads/master/as.csv"
 
 // The first columns of as.csv, the ones the generator reads. It reads past
 // the columns after them, such as the country code.