survive a transient lookup while assembling the sysroot

The gate failed on name resolution for the package archive after apt had
already succeeded in the same job -- intermittent, not configuration.

scripts/make-aarch64-sysroot.py

@@ -23,6 +23,7 @@ import re
 import shutil
 import subprocess
 import sys
+import time
 import urllib.request
 
 MIRROR = "http://ports.ubuntu.com/ubuntu-ports"
@@ -55,11 +56,20 @@ SKIP = {"gcc", "gcc-13", "libgcc-13-dev", "libc-dev", "libc6-dev-arm64-cross",
         "pkg-config", "pkgconf", "debconf", "dpkg", "install-info"}
 
 
-def fetch(url):
-    # The archive rejects urllib's default agent with 403.
+def fetch(url, attempts=4):
+    # The archive rejects urllib's default agent with 403. The retry is for
+    # transient name resolution: a build that reconstructs 155 packages over
+    # the network should survive one bad lookup rather than failing the gate.
     req = urllib.request.Request(url, headers={"User-Agent": "viewtop-sysroot/1"})
-    with urllib.request.urlopen(req, timeout=120) as r:
-        return r.read()
+    for attempt in range(attempts):
+        try:
+            with urllib.request.urlopen(req, timeout=120) as r:
+                return r.read()
+        except urllib.error.URLError as exc:
+            if attempt == attempts - 1:
+                raise
+            print(f"  retrying {url.rsplit('/', 1)[-1]} after {exc.reason}", file=sys.stderr)
+            time.sleep(3 * (attempt + 1))
 
 
 def load_index(suite, component):