Avoid hanging endlessly on unreachable cache urls

When a download redirector redirects to a broken mirror, timeout
quickly rather than waiting until the overall job is being timed out.

Change-Id: If7eb63d406aaf61f71aa9203cf708c474aa63fd0

diskimage_builder/elements/cache-url/bin/cache-url

@@ -86,7 +86,7 @@ else
     success="Downloaded and cached $url for the first time"
 fi
 
-rcode=$(eval curl -v -L -o $tmp -w '%{http_code}' $curl_opts $url $time_cond)
+rcode=$(curl -v -L -o $tmp -w '%{http_code}' --connect-timeout 10 $curl_opts $url $time_cond)
 if [ "$rcode" == "200" -o "${url:0:7}" == "file://" ] ; then
     # In cases where servers ignore the Modified time,
     # curl cancels the download, outputs a 200 and leaves